DXR.1 Digital Auio Coec SECTION 1...INTRODUCTION... 3...DIGITAL SERVICES... 3...WHAT COMES WITH THE DXR.1?... 3 2...SETUP... 4...DATA CONNECTION... 4...POWER CONNECTION... 4...AUDIO CONNECTIONS... 5...CONTACT CLOSURE... 5...V.35 AND X.21 CONNECTIONS... 6...LEVEL ADJUSTMENTS... 7...TERMINAL EQUIPMENT... 8 3...INTERNAL SETTINGS... 9 4...TROUBLE SHOOTING... 11...DXR.1 LOOPBACK... 11...TERMINAL ADAPTER OR CSU/DSU LOOPBACK... 12...REMOTE LOOPBACK... 12 5...TECHNICAL DETAILS... 14...G.722 ALGORITHM... 14...THEORY OF OPERATION... 14...SYNCHRONIZATION... 15...ISDN AND SWITCHED 56... 16...INVERSE MULTIPLEXING... 17...MIX-MINUS... 18 6...APPLICATIONS... 20...NETWORK DISTRIBUTION... 20...AUTO RECORDING... 20...AUTOMATION TONES... 20 7...GLOSSARY... 22 SCHEMATICS
Comrex Corporation About Comrex Comrex has been builing reliable, high quality broacast equipment since 1961. Our proucts are use aily in every part of the worl by networks, stations an program proucers. Every prouct we manufacture has been carefully esigne to function flawlessly, uner the harshest conitions, over many years of use. Each unit we ship has been iniviually an thoroughly teste. Most items are available off-the-shelf, either irectly from Comrex or from our stocking ealers. Comrex stans behin its proucts. We promise that if you call us for technical assistance, you will talk irectly with someone who knows about the equipment an will o everything possible to help you. Our toll free number in North America is 800 237-1776. Prouct information, along with Engineering Notes an User Reports, is available through our website at www.comrex.com. Our email aress is info@comrex.com. Warranty an Disclaimer All equipment manufacture by Comrex Corporation is warrante by Comrex against efects in material an workmanship for one year from the ate of original purchase, as verifie by the return of the Warranty Registration Car. During the warranty perio, we will repair or, at our option, replace at no charge a prouct that proves to be efective, provie you obtain return authorization from Comrex an return the prouct, shipping prepai, to Comrex Corporation, 19 Pine Roa, Devens, MA 01434 USA. For return authorization, contact Comrex at 978 784-1776 or fax 978 784-1717. This Warranty oes not apply if the prouct has been amage by accient or misuse or as the result of service or moification performe by anyone other than Comrex Corporation. With the exception of the warranties set forth above, Comrex Corporation makes no other warranties, expresse or implie or statutory, incluing but not limite to warranties of merchantability an fitness for a particular purpose, which are hereby expressly isclaime. In no event shall Comrex Corporation have any liability for inirect, consequential or punitive amages resulting from the use of this prouct. Copyright Notice The Comrex proprietary coe an license, thir party proprietary coe resiing in an otherwise associate with this prouct are protecte by copyright law an international treaties. Unauthorize reprouction or istribution of this prouct, or any portion of it, may result in civil an criminal sanctions, an will be prosecute to the fullest extent of the law. US Government Restricte Rights - Use, uplication, or isclosure by the US Government is subject to restrictions set forth in subparagraph (c)(1)(ii) of the Rights in Technical Data an Computer Software clause at DFARS (48 CFR) 252.227-7013 or subparagraphs (c)(1) an (2) of the Commercial Computer Software-Restricte Rights clause at FAR (48 CFR) 52.227-19, as applicable. 2
DXR.1 Prouct Manual SECTION 1. INTRODUCTION The DXR.1 is a igital auio compression evice esigne for broacasters. It employs the ITU G.722 international stanar to sen an receive 7.5 khz auio at transmission rates of 56/64 kbps. This unit also provies a full 15 khz biirectional fee on 112/128 kbps circuit. DIGITAL SERVICES The DXR.1 works on any synchronous 56, 64, 112 or 128 kbps igital transmission system, incluing ISDN, Switche 56, Digital Data Service (DDS), Fractional T1, igital satellite links an wireless moems. For part-time auio fees, ial-up Basic Rate Installation (BRI) ISDN or Switche 56 (SW56) igital lines are ieal since they are bille at a low monthly cost, plus per minute usage which is not much more than stanar analog lines. BRI ISDN is available throughout the worl. SW56 is a North American precursor to ISDN an both services work with one another. Deicate 56 kbps or 64 kbps telephone lines (point to point) have been available for quite some time. They may provie a significant cost savings for applications that require full time or near full time exchange of auio. In North America, this is known as Digital Data Service (DDS). Sometimes, Fractional T1 (or E1) is available. This service provies a eicate link with higher ata rates. Portable satellite earth stations an wireless moems may also be use with the DXR.1. Comrex has an extensive engineering note library escribing these applications. Contact us for further etails. Note: External terminal equipment is necessary to connect the DXR.1 with the ata channel. Please refer to page 8 for more information on terminal equipment. WHAT COMES WITH THE DXR.1? The following items are shippe with a new DXR.1: (1) A/C power cor (1) DB25 male to DB25 female cable (1) Operating manual (1) Warranty car (Please fill out an return.) 3
Comrex Corporation SECTION 2. SETUP DATA CONNECTION This connection is mae between the Terminal Aapter or CSU/DSU an the DXR.1 back panel. This is one through the DATA IN/OUT connector. The DB25 (25 pin) connector using the EIA530 protocol is the stanar ata port. Assuming your terminal equipment uses this same connector, you will nee a 25 pin straight-through cable (provie by Comrex) to connect to this port. The pin-out of the DB25 connector is: Pin # Function 1 Shiel 2 TX Data A 3 RX Data A 7 Groun 9 RX Clock B 12 TX Clock B 14 TX Data B 15 TX Clock A 16 RX Data B 17 RX Clock A 20 DTR A 23 DTR B 24 is reserve. Do not connect to it. If your terminal equipment uses a protocol other than EIA530, a converter cable will be require. Optional aapter cables for V.35 an X.21 connections are available through Comrex. If you want to construct your own aapter cables, the pin-outs are on page 6. Note: Simply because terminal equipment uses a 25 pin D connector oes not mean that it uses EIA530. Implementations of this sort vary wiely. Check that the equipment specifically mentions EIA530 or RS530 as the protocol for its ata port. Otherwise, it will not work. POWER CONNECTION The power connection is on the back panel of the DXR.1. The unit is switch selectable for either 115 VAC/60 Hz or 230 VAC/50 Hz. The switch is locate on the insie of the unit, on the printe circuit boar an is shippe from the factory in the 115VAC/60Hz position. To change the setting, first make sure the unit is unplugge. Open the DXR.1 case by removing the four screws on each en of the unit an the ten screws on the top. The power selection switch is locate irectly behin the power connection. For 230 VAC/50 Hz, 230VAC will be reveale. You must also change the fuse from a 160 ma slo-blo fuse to an 80 ma sloblo fuse. 4
DXR.1 Prouct Manual To change the fuse, open the cover on the fuse box irectly to the right of the power connection. Turning the cover 1/4 turn will allow you to access the fuse. Make sure the correct power is selecte. The wrong ecision can estroy your unit. AUDIO CONNECTIONS Connect your fee to the AUDIO IN plug. This connection is mae through a 3-pin XLR female plug with the following pin-outs: Pin # Function 1 Groun 2 Balance auio high 3 Balance auio low It is best when using professional auio gear to connect everything together using balance auio connections. Sometimes, however, it is unavoiable to connect to consumer equipment with unbalance connections. If you must use unbalance auio, here's how to connect to the DXR.1: Inputs: Pin 2 to auio high, pins 1 an 3 to groun Outputs: Pin 2 to auio high, pins 1 an 3 to groun The AUDIO OUT is via a 3-pin XLR male connector with the same pin-outs as the AUDIO IN connector. Note: The coec provies two istinct an separate channels. You are transmitting to the receive en through the AUDIO IN XLR connector an the receive en may be feeing cues or other programming back to you through the AUDIO OUT XLR connector. Therefore, the AUDIO OUT channel is use to hear what is being sent to you, not what you are sening. For more on this, see the Applications section on pages 20-21. CONTACT CLOSURE A CONTACT CLOSURE jack is locate on the back panel. This is available for controlling other equipment, such as starting a tape recorer or turning on certain cue lights. The contact closure is a 1/8 mini jack, an the contact is complete when the READY LED lights up. More information on the use of this jack is available in the Applications section on pages 20-21. The contact closure connection is not esigne to switch high power connections. DO NOT connect AC power through the contact closure connection. The switch is esigne to carry low voltage (5-15V at low current less than 50 ma). If this connection is require to switch a high power evice (on-air light, selenoi, etc.), buffer it externally with a low control voltage DC relay. 5
Comrex Corporation V.35 AND X.21 CONNECTIONS Pin Connections for Comrex EIA530 (DB-25) to V.35 Cable EIA530 (DB-25F) Signal V.35 (34-pinM) 1 Shiel A 2 Transmit Data A P 3 Receive Data A R 7 Groun B 9 Receive Clock B X 12 Transmit Clock B AA 14 Transmit Data B S 15 Transmit Clock A Y 16 Receive Data B T 17 Receive Clock A V 24 Data Terminal Reay H Pin Connections for Comrex EIA530 (DB-25) to X.21 Cable EIA530 (DB-25F) Signal X.21 (DB-15M) 2 Transmit Data A 2 20 Data Terminal Reay A / Control 3 3 Receive Data A 4 6 Data Set Reay A / Inication 5 15, 17 Clock A 6 7 Groun 8 14 Transmit Data B 9 23 Data Terminal Reay B / Control 10 16 Receive Data B 11 22 Data Set Reay B / Inication 12 9, 12 Clock B 13 1 Shiel, Groun Shell 6
DXR.1 Prouct Manual LEVEL ADJUSTMENTS With auio being fe into the system, look at the PEAK light on the front panel. This re light shoul be flashing occasionally. There is no set pattern for the flashing, but if it is flashing about 15% of the time, your auio shoul be receive in goo fashion. If the light is on constantly, your auio will probably soun clippe an istorte. If the light is not on at all, the level you are sening will be too low, an you may have noise problems. The solution to both of these problems is to ajust the level. The ajustment is one on the equipment feeing the auio mixer, tape machine, etc. The DXR.1 is shippe with the auio input an output level set to 0 Bu. If require, it may be ajuste to either -10 or +10 Bu. This is one via internal jumpers. Refer to the Internal Settings section on pages 9-10 for more etails. On the DXR.1 there is no nee to select the ata rate you will be using. The DXR.1 automatically senses the ata rate an ajusts the frequency response to match. When sening 56/64 kbps the frequency response will be 7.5 khz. At 112/128 kbps, the frequency response will be 15 khz. To insure the DXR.1 is working properly an will give you no problems when you make your connection with the receive en, you can run a series of tests. These are calle loopback tests because the auio you are feeing into the system is fe back to you through a loop. The majority of igital equipment provies the capability of loopback testing to isolate problems that may occur. See the Trouble Shooting section on pages 11-13 for more information. Now the DXR.1 is connecte an setup. You are reay to communicate with the other en of your link, so place the call through your Terminal Aapter or CSU/DSU. Once your DXR.1 an the coec on the other en have establishe contact, the green READY light on the front panel will light up. At this point, you may begin transmitting an receiving auio. 7
Comrex Corporation TERMINAL EQUIPMENT Whatever type of igital telephone channel you use, you will nee some sort of evice to link the DXR.1 to the ata channel. For ISDN, this is known as a Terminal Aapter (TA) an for Switche 56, it is known as a Channel Service Unit/Data Service Unit (CSU/DSU). We are happy to recommen a specific TA or CSU/DSU that is appropriate for your application. At the minimum, any terminal equipment you choose shoul have: Synchronous ata port capable of V.35, RS530, X.21, or other balance ata protocol Dial pa Local an remote loopback capability V.120 rate aaptation (use for 56/64 conversion) Other nice but not require features: Dual ata ports (for Terminal Aapters) RS232 remote ialing For Terminal Aapters, BONDING or IMUX capability (require for 15 khz transmission) Memory ial With ial-up igital networks the receiving en of the call answers automatically. This allows unattene operation at one en of the link. 8
DXR.1 Prouct Manual SECTION 3. INTERNAL SETTINGS The DXR.1 has been factory configure to meet the majority of applications. However, in this section, we provie information on settings which may be change to meet special nees. These settings inclue: Auio Input/Output Levels Clock Loopback Data Rate Moe Select Clock Select Changes are mae via internal jumpers on the main PC boar. In orer to access these jumpers, you must remove the top cover of the chassis. Please remember to isconnect power from the DXR.1 before removing the cover. To change the setting, remove the hoo that fits over the pins on the jumper block, an then slie it over the new pin settings. Auio Input (J5) / Output Levels (J6) The auio input an output levels are factory ajuste to 0 Bu. If require, it can be moifie to either -10 or +10 Bu. This is accomplishe by moving the hoo on jumpers J5 an J6. J5 J6 9
Comrex Corporation J9 J7 J8 Clock Loopback Data Rate (J9) The DXR.1 automatically etects the clock rate coming from the Terminal Aapter. For loopback testing purposes, the DXR.1 uses 56 kbps which transmits ata at 7.5 khz. If it is critical that you hear your loopback test at 15 khz, the jumper may be move to the 112 kbps position. There is normally no nee to move this jumper. Moe Select (J7) The DXR.1 automatically etects the ata rate an ajusts the auio banwith being sent. However, it is possible to harwire the chosen ata rate. Moving the jumper to the center position, selects 56/112 kbps. The opposite en selects 64/128 kbps. Clock Select (J8) This jumper allows the transmit/receive clock to be change. It is factory configure to sample the RX clock but may be move to the TX position to sample the TX clock. This moification is not usually require. 10
DXR.1 Prouct Manual SECTION 4. TROUBLE SHOOTING The Comrex DXR.1 coing algorithm eliminates reunancy in auio. For this reason, the DXR.1 cannot be subjecte to traitional specifications of istortion an signal-to-noise ratio. Most tests one with the coec shoul be by subjective listening between the original source material an coecprocesse auio. Because of the algorithm's ynamic processing properties, tests one with tones ten to prove little. Unlike analog technology, which might work but just be a little off, igital technology tens to either work perfectly or not at all! The trick is to isolate the source of the problem to either the telephone network or the equipment attache to it, so you know where to turn for a solution. We will start with testing the first link in the chain, the DXR.1, then the Terminal Aapter or CSU/DSU an finally the network. DXR.1 LOOPBACK This test examines the coec inepenent of other equipment. Auio must be fe into the unit through the AUDIO IN plug, an you must be able to monitor the auio coming out. Power must be connecte to the DXR.1. Locate the CLOCK AND DATA LOOPBACK switches on the rear panel. Move both of these switches to the up position. This activates two things. It puts the local clock into use to rive the signal, an it connects the encoe an ecoe channels. DXR.1 Loopback DXR.1 TA or CSU/DSU TA or CSU/DSU DXR.1 The READY light on the front panel will be green, an you will hear the same auio that you are feeing into the DXR.1. If you o not hear auio, or it is istorte, check your connections. Make sure the power light on the front panel is ON an the peak light is flashing occasionally. If there is still a problem, please contact Comrex to arrange for repair of the DXR.1. 11
Comrex Corporation TERMINAL ADAPTER OR CSU/DSU LOOPBACK Check the manufacturer s irections for exact instructions on engaging the loopback function. This may be calle local or DTE loopback. Move both loopback switches on the DXR.1 to the normal (own) position. This allows the CSU/DSU or Terminal Aapter to control the testing. The auio will be going from the source, through the coec an coming back to the coec from the Terminal Aapter or CSU/DSU without ever getting on the network. TA or CSU/DSU Local Loopback TA or TA or DXR.1 DXR.1 CSU/DSU CSU/DSU The auio you hear through AUDIO OUT shoul be the same as what you are feeing into the system, an the DXR.1 READY LED shoul be lit. If the coec checke out fine in local test, an the Terminal Aapter or CSU/DSU local loopback fails, the problem is with the Terminal Aapter, CSU/DSU or cable. Contact the manufacturer for assistance. REMOTE LOOPBACK Remote loopback allows auio to be sent own the network, receive by the far en Terminal Aapter or CSU/DSU an returne to you. This test will check out the network but requires the capability of remote loopback on your Terminal Aapter or CSU/DSU. The terminal equipment must be of the same type (both ISDN, both SW56, etc.) on either en of the connection. If the types are incompatible, remote loopback will not be possible. Make sure both switches on the DXR.1 are in the normal (own) position before you begin. TA OR CSU/DSU Remote Loopback DXR.1 TA or TA or DXR.1 CSU/DSU CSU/DSU If you can hear your own auio with remote loopback engage an the READY light is lit, then you know for sure the DXR.1, the Terminal Aapter or CSU/DSU an the network, as well as the remote Terminal Aapter or CSU/DSU are working fine. 12
DXR.1 Prouct Manual Of course, you are always welcome to contact us if you encounter a failure of the system. However, please remember that if the failure lies either in the Terminal Aapter or CSU/DSU, the cable or the network, there is little Comrex can offer in terms of assistance. Most manufacturers of igital equipment have iagnostic tests built into their equipment similar to those we have illustrate. In aition, most igital service carriers have customer hotline numbers for you to report a problem an help you locate the trouble spot on the igital network. We suggest you keep a list of these numbers hany for the occasions when you run into trouble. 13
Comrex Corporation SECTION 5. TECHNICAL DETAILS G.722 ALGORITHM The coec is a system that encoes an ecoes auio signals for transport over igital networks. At the transmit en, the information is encoe an it is ecoe at the receive en. Simple. Well, not so simple. As with most things in the worl, if everyone create their own metho of oing things, nothing woul work together. Something as simple as the stanarization of power plugs means that we on't think twice about buying appliances or electronic components. But we o think twice about what format our vieotape is in VHS or BETA. But at least it is a small fiel from which to choose. The same thing is happening with the compression algorithms use to encoe an ecoe auio signals. International stanars boies have forme to create stanars. There are ifferent stanars available (such as VHS an BETA), an it is up to you to select which one you will implement. It is also up to you to insure that the venor you select is implementing the stanar with no changes (that can mean your equipment will not work with other manufacturers equipment, an you will be boxe into a corner). The international stanar known as CCITT G.722 specifies the algorithm that coecs use to convert analog to igital signals an vice versa. The DXR.1 follows this stanar very carefully an can communicate with G.722 coecs from other manufacturers. THEORY OF OPERATION The Comrex DXR.1 performs a igital algorithm in real time on sample igital input auio. The unit is base on two high spee computer chips, known as igital signal processors (DSPs). The iea behin the coec (an any other DSP-base evice) is to perform functions on analog signals which have been ivie into samples taken at iscrete times. These samples are then quantize (assigne a fixe value) an fe as a stream of binary numbers into the DSP. The basic assumption of the coec is that igitize auio contains more information than is neee to reprouce it in analog form. By eliminating this reunant information, more auio information may be store or transmitte. As mentione before, the input to the DSP portion of the coec is a series of iscrete time samples. Each portion of the coec link (transmitter an receiver) contains a computer program which can preict the next sample base on previous values processe. This function is performe ientically in the transmitter an receiver. The ifference between the transmitter an 14
DXR.1 Prouct Manual receiver is that only the transmitter knows the true value of the next iscrete time sample. Since it alreay possesses an approximation of this value, it can calculate the ifference between the two numbers it possesses. This ifference is what the transmitter sens to the receiver. The receiver uses this ifference to calculate the true value. Since the ifference signal contains less than the ata sample, ata rate is conserve. In human speech (an most other auio), much more energy exists in the lower part of the auio spectrum than in higher frequencies. Therefore, the coec reprouces auio more accurately at the lower en than at the higher en. Using igital filters, auio is ivie between high an low sub-bans, an each sub-ban is sent through the encoer-ecoer combination separately. The lower ban can then use up the majority of the bits available, leaving only a few for the relatively less complicate high ban. Discrete time sampling an quantization of an analog waveform are known as Pulse Coe Moulation (PCM), since the coec algorithm uses ifferences between samples. The preictors aapt automatically with changing values of previous input samples, so we call the algorithm use Aaptive Differential PCM, or ADPCM. When we a the concept of iviing an conquering iniviual bans, the process becomes Sub-ban (SB) ADPCM. SB-ADPCM is efine as an international stanar by the CCITT as recommenation G.722. The text of this specification is public information an is a goo source for further information on this algorithm. SYNCHRONIZATION The transmitting coec forms its outgoing ata into wors, each consisting of seven or eight characters. The receiving coec is able to ecoe an ecompress ata intelligibly because it has ientifie the beginning an en of each wor it receives. This process of ientifying an aligning with the correct wor orer is calle synchronization. The DXR.1 uses a self synchronizing technique which allows the encoer to use the entire channel for auio ata. With no overhea for synchronization ata, the ecoer can etermine the sync position by performing an algorithm on the raw, incoming ata. It takes about one secon for the ecoer to fin sync an begin ecoing ata. The READY light on the front panel is an inication that the ecoer is in sync. Every half secon, the DXR.1 re-checks to make sure that it is still in sync. If the network causes an error that makes the ata stream line up ifferently, the coec can etermine this an re-sync within one secon. Remember, because the DXR.1 is fully uplex, it is simultaneously encoing/compressing outgoing information an ecoing/ecompressing incoming information. 15
Comrex Corporation ISDN AND SWITCHED 56 Throughout this manual, we refer to two ata rates: 56 an 64 kbps. These rates an multiples of them are the most common you will experience with igital telephone services. When you place a normal phone call on the public telephone network, the link is establishe in a particular pattern. It is an analog signal from you to the central office. (The central office is a telephone facility locate at the en of your local loop. It usually contains a switch connecte to trunks.) A 64 kbps ata link is establishe between the central office nearest you an the one nearest the calle party. From the central office nearest the receiving en, the information is converte back to analog for elivery to the calle party. This type of transmission is perfect for voice communication because the banwith your voice prouces is very narrow. But as the electronic age progresses, higher banwiths are neee to relay information such as pictures an high quality auio. The igital portion of the phone line has to be extene to the user. Most igital telephone circuits are base on the concept of using this same igital telephone network channel an extening the igital portion to the user, eliminating the analog section. Basic Rate Interface Integrate Services Digital Network (BRI ISDN) provies the user with access to two of these channels, which are multiplexe onto the same pair of wires an sent to the user. These two channels (calle B channels) are completely separate. They may be iale inepenently an may be use for voice or for ifferent types of ata transmission. In North America, some of the public telephone networks use a form of signaling which limits the user banwith to 56 kbps channels. In this case, both ens of the telephone link must set their equipment for this lower ata rate. In some areas, the 56 kbps (or multiples of it) setting may be safer, allowing all calls to complete properly regarless of where they are locate. Also in North America, since ISDN has been slow to proliferate, simpler capabilities are available in a service calle Switche 56. As the name implies, Switche 56 is limite to 56 kbps, but can inter-operate successfully with an ISDN line set for this spee. Multiple Switche 56 lines woul be require to achieve higher ata rates. Digital Data Service (DDS) has been available for quite some time in North America. This service provies a point to point eicate 56 kbps or 64 kbps telephone line. DDS may yiel significant cost savings for applications that require full time or near full time exchange of auio. Fractional T1 (or E1), a eicate link capable of higher ata rates (384 kbps), is sometimes available. Wireless moems an portable satellite earth stations may also be use with coecs. 16
DXR.1 Prouct Manual INVERSE MULTIPLEXING Inverse Multiplexing, or IMUXing for short, souns complicate but is actually quite simple. It means combining two or more lower ata rate channels into one, higher ata rate channel. It is an extremely important concept when working on igital phone lines like Switche 56 an ISDN, as igital transmission channels on these services come in chunks of 56 or 64 kbps. These chunks have very little to o with each other normally. They may be route ifferently throughout the telephone network an incur substantially ifferent transmission path elay. Even the two B channels of a Basic Rate Interface ISDN installation offer no guarantee that both calls will be route along the same path. On a North American coast-to-coast linkup, for example, the first B channel connection may be route via Texas an the secon via Michigan. The IMUX must be able to measure the time elay between the two igital channels an elay the fastest so that it arrives synchronously with the slowest. This proceure is calle aggregation an is performe ifferently with ifferent IMUX protocols. When using BRI ISDN, you will fin that several reasonably price Terminal Aapters have an IMUX built into them, usually using a protocol calle BONDING. These IMUXes work quite well an may be use with the DXR.1 to sen 15 khz over ISDN lines. 64 kbps Inverse Multiplexing 64 kbps 128 kbps 17
Comrex Corporation MIX-MINUS Even the simplest remotes are a two-way process. The remote site must sen its auio to the stuio an receive a return fee to monitor the programming. This return fee may be one over a raio station s regular transmitter (with an AM or FM raio at the remote), a special raio link or a telephone circuit. This fee may just go to heaphones at the remote, an it may also be put on speakers for the local auience. The problem comes when there is a time elay in getting auio to an/or from the stuio. In this case, the remote talent hears a elaye version of their voice in the heaphones an may fin this very istracting. Even a remote one with simple equipment or a frequency extener on plain phone lines may have this problem on a long-istance call. All remotes using ISDN, Switche 56 an POTS coecs will have elays each way as signals are processe from analog to igital, compresse, uncompresse an converte back to analog auio. Some igital compression schemes, such as G.722, result in shorter elay times, but there will still be a reverb effect in heaphones at the remote site, if their auio is sent back from the stuio. In any of these cases, it may not be possible for the remote people to listen to an off-air or program channel fee. The solution is mix-minus. A mix-minus fee has a mix of all of the programming on the raio station (or network) minus the auio from the remote. In other wors, the station or network oesn t sen the remote auio back to the remote. At the remote en, this mix-minus fee is converte back to an air monitor by mixing in the local auio from the remote. For raio stations, in aition to fixing the time elay problem, using a mixminus fee has two other avantages. First, if the station uses a 6-7 secon elay to allow eiting of phone calls, pre-elay auio can be sent to the remote site. Secon, if there is a PA system at the remote, they will be able to run the speaker levels higher with the mix-minus auio. This is because the remote microphone auio is not running through the station s auio processing, an the levels stay uner the control of the remote operator. The simplest way to o one mix-minus fee in a typical raio stuio is to use the Auition or secon program channel. On many auio consoles, each faer s output may be sent to both Program an Auition. If your boar will allow those fees simultaneously, just set all of the moules to Program an Auition, with the exception of the one carrying the remote auio. Set that one to Program only. The Auition channel will then be a mix of everything on the console except the remote. That will be your mix-minus, an it shoul be sent to the remote site. 18
DXR.1 Prouct Manual One caution make sure that auio is being sent to an from any telephone moules you may have in the console. They may have been esigne to work with only one channel at a time, either Program or Auition, but not both. If so, you will have to check with your tech guy or the boar manufacturer for avice. If you use multiple auio coecs, you shoul investigate the Comrex Mix-Minus Brige. This will allow you to expan one Program/Auition setup to hanle five coecs or other remote auio evices. It also provies IFB (talkback) to remote sites. Main program fee leaving stuio Auition Output (all auio except coec) TO Remote In AUDIO CODEC Out Coec Output (remote auio ) FROM Remote P P P P P P P P P PGM OUT AUD OUT A A A A A A A A A MIC 1 MIC 2 CART 1 CART 2 TEL TAPE CD REM NET "I M USING MIX-MINUS, AND I STILL HEAR AN ECHO! If you are oing a call-in talk show on the roa, the remote people may complain of hearing an echo when a caller is put on the air. With the telephone pot own, everything is OK. The culprit is the telephone hybri being use to put callers on the air. Some of the remote auio is leaking through the hybri an mixing with the caller auio. Moern igital hybris o a much better job of preventing this than the oler units that ha to be manually tweake for each call. If you are using a igital hybri an having this problem, ig out the manual an reo the hybri s initial setup. 19
Comrex Corporation SECTION 6. APPLICATIONS The DXR.1 provies a cost saving alternative to satellite fees or eicate circuits. They are ieal for applications requiring high quality (7.5 or 15 khz) mono auio. There is very little processing elay, making the DXR.1 a goo choice for live programming such as talk, news an sports, where real-time cueback is essential. NETWORK DISTRIBUTION Sports reports are often recore an mae available to many stations. Each station ials into the servicing station through their own coec. Once the receive an transmit coecs have gone through a hanshaking ritual to insure they are compatible, the DXR.1 READY light on the transmit en will light up. Through the use of the contact closure jack, a tape recorer is starte, an the sports report is transmitte to the receiving station. Once the connection is broken, the switch is opene, an the tape recorer is returne to reay state waiting for the next call. AUTO RECORDING The same technique can be use to recor information. Reporters in the fiel may complete stories at any hour of the ay. By calling into the DXR.1, the hanshake between coecs is accomplishe an the recorer is starte via the contact closure jack. This provies for unattene gathering of stories for either irect broacast or eiting at a later ate. AUTOMATION TONES Broacast networks sometimes use sub-auible tones in the range of 20 to 50 Hz to allow affiliates to automate their network cutaways an rejoins. 25 Hz is a popular frequency, an the combination of low-frequency roll-off in the DXR.1 an our customer s equipment may provie an output too low for a tone ecoer expecting full line level. Since the G.722 processing in the DXR.1 favors the lower auio frequency ban by allocating more bits in the coing process, the automation tones are hanle very well in the igital processing. There are two points in the DXR.1 analog section, however, that will restrict the low frequency performance. Referring to the DXR.1 boar schematic inclue at the en of the manual, capacitor C4 couples the output of U19D to resistor R12, which sets the input impeance to U19C. The factory-supplie value of C4 is 470nF (0.47mF). Increasing this value to 2.2mF, a commonly available value for non-polarize capacitors, will improve the low frequency response of the encoer section. Larger values are fine, but we recommen a non-polarize unit. In the ecoer, a change woul only have to be mae if the DXR.1 auio output was terminate in a true 600 ohm loa. If so, the 22mF output coupling capacitors, C32 an C33, which are connecte to U24, can be eliminate by aing a jumper across each of them. 20
DXR.1 Prouct Manual Just be aware that you may present a small -c offset voltage to your equipment. The alternative woul be larger non-polarize electrolytic capacitors esigne for auio use. If the DXR.1 is feeing a briging loa (>5K ohms, typical of most console inputs), the 22mF capacitors may be left in place. If you make these changes an are still having problems, you shoul check that the automation tone level at the DXR.1 input is actually up to the program line level. Small auio transformers in any equipment you may have between the tone generator an the DXR.1 may either reuce the level of the tones or a istortion proucts. Many transformers will not hanle 25 Hz tones at line level, espite the manufacturer s claims. The same reasoning applies at the receiving en in any equipment between the ISDN coec an the automation tone ecoer. 21
Comrex Corporation SECTION 7. GLOSSARY CSU/DSU DATA PORT DATA RATE DIGITAL VS. ANALOG DSP DTE/DCE Channel Service Unit/Data Service Unit. On some ata networks, these are two separate evices. On most networks use with the coec, this is a box which sits between the coec an the ata circuit, use to interface an conition the ata coming on an off the network. This box may also contain iagnostic testing functions an inicators, an in the case of switche services, will perform all your ialing functions. A CSU/ DSU is require for all SW56 an DDS circuits an is not inclue with your coec. The physical an electrical protocol use by the coec an the TA or CSU/DSU to transfer ata between each other. Analog transmission meia is specifie in banwith (usually in Hertz) an signal to noise (usually in B). Since the principles behin igital transmission are so ifferent, meia are specifie in ifferent parameters. Rather than how much analog information is passe, a igital user is concerne with how many bits per secon can be sent own the channel. An analog electrical signal (or soun or light, etc.) is note by a funamental change in character with respect to the information being conveye. For example, an AM raio station changes the amplitue of a carrier signal to varying egrees epening on the amplitue of the music it is carrying. A igital signal is always in one of two states (on or off), but varying at a rate fast enough that information encoe into numbers (quantize) can be transferre. Another way to istinguish analog from igital is that an analog signal has an infinite number of egrees of changes which convey information. A igital signal has only two. One of the largest avantages of igital transmission is that as long as a receiver can istinguish between the two states in the signal, noise will have no effect on it. Digital Signal Processing. The concept of sampling analog waveforms in iscrete time an manipulating these samples using algorithms which woul be ifficult or impossible in the analog omain. Data Terminal Equipment/Data Computer Equipment. To avoi confusion, the ata protocols mentione above esignate equipment an ports as either DTE or DCE. The TA or CSU/DSU is ALWAYS the DCE, an the coec is ALWAYS the DTE. Plugging two DTEs together will not establish communication between them, since the DCE provies all the clocks require to run the ata aroun. 22
DXR.1 Prouct Manual DUPLEX ISDN LOOPBACK NOISE On plain ol telephone service (POTS), the auio transmission can be consiere half uplex, since if both parties speak at the same time, their voices will intercept on the single pair of wires on each en of the call. Most igital systems are uplex or 4 wire, allowing simultaneous an inepenent ata (or encoe auio) to pass in each irection. Some systems may be simplex which pass igits only in one irection. Integrate Services Digital Network. The worlwie stanar for igital telephony. ISDN actually escribes a complex set of international stanars concerne with igital telephony. This service is currently available in two configurations. Basic Rate Interface (BRI) provies the user with two inepenent 64 kbps switche channels. Primary Rate Interface (PRI) allows user ata rates approaching those of T1 (1.54 Mb/s). Analog signals are easy to test. One simply probes the point of interest with an oscilloscope an checks for the proper signal. High spee igital signals complicate things because they can t be measure easily by traitional test equipment. To make tests easier, igital equipment often comes with loopback capability. Borrowe from the telephone repairmen who ha no wish to trace miles of circuits, loopback iagnostics allow you to instruct a piece of equipment in your igital link to echo any information sent to it in the reverse irection. When properly loope back, the coec shoul echo (from the receiver output) any auio sent into the transmitter. By enabling loopback at ifferent points on a network (i.e. TA or CSU/DSU, coec, remote TA or CSU/DSU loopback) the efective portion can more easily be etermine. In this application, noise is anything present between the coecs other than the binary signal being transmitte. Digital signals are by nature immune to noise until the noise level approaches the point where a 1 looks like a 0 an vice versa. Digital circuits are susceptible to two ifferent type of noise. The first type is just backgroun noise which occasionally reaches a sufficient level to cause bit errors. The other type, known as burst errors, cause incorrect ata to be sent for a istinct amount of time. Bit errors cause little noticeable egraation to ADPCM auio, ue to the nature of the algorithm. Burst errors cause severe istortion an may cause loss of synchronization. 23
Comrex Corporation PCM/ADPCM SWITCHED 56 SYNCHRONIZATION TERMINAL ADAPTER (TA) Pulse Coe Moulation (PCM) is the technique use by CD players an other evices to igitize auio. The coec converts PCM to Aaptive Differential PCM in orer to conserve meia banwith. In North America, this service is very similar to DDS but allows the user to place calls between several points an cut costs by only paying for part time service. Consiere to be a precursor to ISDN. Many ata networks simply provie a bit stream to the user, without any information on how to ivie these bits into wors. Some networks use overhea bits to etermine the start an stop of each wor. If the coec is to give you the best possible use of your ata channel, it can t affor the overhea require to provie alignment information, if it isn t provie by the network. For this reason, the coec s receiver takes in raw, unframe ata an analyzes it to etermine the proper wor alignment. This auto synchronization scheme allows the receiver to frame wors with no other information. In the case where you are communicating with a less sophisticate coec (from another manufacturer), the transmitter may be configure so that output wors line up with network timing information, if available. This assures that the other coec will receive wors aligne with the network timing. Uner most circumstances this won t be necessary or esirable. This can be thought of as a CSU/DSU for an ISDN line. Its function is actually to aapt non-isdn equipment to the ISDN user rate. It may also provie you with a choice of bearer services, which etermine the type of ISDN call you will make. For the most part, the coec requires the placement of a 56 or 64 kbps clear channel ata call. 24