6.7 Compatibility Issues in Audio Interfacing

Despite the many standards relating to audio interfaces, or perhaps because of them, it is possible that practical difficulties may arise when attempting to interconnect two or more devices. The problem of 'getting devices to talk to each other' is possibly less serious than it was when this book was first written, but there are still areas of difficulty and the number of interface types is large. Communication problems may usually be boiled down to one of a few common sources of incompatibility . Not only must the user know how to work around basic incompatibilities, but also it is necessary to be aware of the possibility for incorrect communication. Devices may be made to 'talk' but something may be lost or gained in the translation!

The majority of this section is devoted to the standard two-channel interface. Communications between devices using dissimilar manufacturer-specific interfaces will nearly always require the use of a format convertor such as those described in section 6.12.1. In addition to the extended discussion of practical interfacing contained in the main text, there is a reference 'troubleshooting guide' to audio interfacing at the end of this section. If problems are encountered with communication over computer networks one normally needs to deal with them as computer networking problems rather than audio interfacing problems. This may involve correct configuration of routers, IP addresses, network drivers and ports, among other things. This is a topic in its own right, about which much is written, and will not be covered further here.

6.7.1 Incompatibilities between Devices Using the Standard Two-Channel Interface

There are only really two reasons why devices using nominally the same interface will not communicate, summed up as either electrical incompatibility or data incompatibility. There is also, of course, the possibility for sampling frequency incompatibility between devices, but this could be considered as a combination of electrical and data incompatibility. If the two devices are using the same electrical interface, such as AES3 for professional purposes or IEC 60958-3 for consumer purposes, there is only a very small chance of electrical mismatch, particularly now that the more recent versions of the standards have limited the options for electrical incompatibility. The more likely cause of any problems is that differences exist between the data transmitted and that expected by the receiver. If direct links are attempted between consumer and professional equipment then there is potential for both electrical and data incompatibility, as discussed below.

6.7.2 Electrical Mismatch in Professional Systems

Between identical interfaces (transmitter and receiver) the most likely electrical problems to arise are (a) loss over long lines; (b) noise and distortion over long lines; (c) impedance mismatch. These can in general be avoided by good system design and by adhering to the recommendations contained in the appropriate standard, but occasionally one encounters a poor electrical installation or needs to make use of existing wiring which may not be ideal for the job of carrying digital audio. When such electrical problems are encountered the most likely symptom is that the receiver will find it difficult or impossible to lock to the incoming data signal, resulting in intermittent operation or indication of 'loss of lock' at the digital input. In practice receivers vary widely in their ability to lock to poor quality signals, and thus it may be found that a signal which works satisfactorily with one receiver proves unsatisfactory with another. Using devices such as those discussed in section 6.9 it is possible to determine the 'health' of the received data signal, as well as examining problems with data.

A receiver conforming to AES3 should be able to decode a signal with a minimum eye height of 200 mV, and since the transmitter normally produces at least 5 volts the resistive cable attenuation has to be quite large before it will reduce the eye height below this value. More problematical than simple resistive loss is high-frequency roll-off over a long cable. This may need to be corrected by using suitable equalization at the receiver (see section 4.3.3), although equalization should be treated with care since it will only work if the line is relatively noise free. (If the line is noisy then equalization may actually make the problem worse and it has been suggested that if such equalization is to be used it perhaps belongs at the transmitter end rather than the receiver.)

High-frequency loss affects the narrow pulses of the data stream before the wide ones and these narrow pulses may either fail to provide a zero crossing or even disappear altogether in extreme cases. The greater the HF loss the more likelihood there is of intersymbol interference and data edge timing jitter, making it more difficult for the receiver to lock to the received data. Dunn ¹ suggests that the cable used should not exhibit attenuation at 6 MHz which is more than 6 dB greater than that at 1 MHz over the distance used, otherwise equalization will be required (this is for basic sampling rates). Operating the interface at higher data rates than the original specification will put greater demands on this criterion, extending it to higher frequencies. Although conventional analog audio cable can and has been used in many cases, it is becoming common for new installations to be wired for digital audio with cable having higher specifications and a characteristic impedance which is better controlled and closer to 110 ohms than conventional microphone cable.

Impedance mismatches were more likely under the original AES3 specification than they became under the 1992 revision, due to the 250 ohm termination impedance specified in AES3-1985. (This was to allow for between one and four receivers to be connected in parallel across one signal line.) Such mismatches may result in internal reflections such that transmitted pulses are reflected from the receiving end to interfere with pulses travelling in the opposite direction, and the cable may begin to function as an antenna both picking up and radiating interference. Now that the termination impedance is specified at 110 ohms and point-to-point interconnects are required, it may be necessary to modify the input impedance of older receivers by fitting a parallel resistor of around 200 ohms so as to make the termination nearer to the correct value. If it is necessary to feed more than one receiver from a single driver it is recommended that suitable digital distribution amplifiers (DDAs) are used, or alternatively one could use passive resistive splitters.

In order to avoid impedance mismatches in between transmitter and receiver it is important that the cable used is consistent along its length and that joins are not made between dissimilar cable types. Problems may also arise if digital audio signals are routed via analog patch bays in which short sections of cabling with mismatched impedances may exist. Any cable 'stubs' in such installations produce short-delayed reflections with fairly high amplitude that can interfere with the transmitted data signal.

Users wishing to carry signals over long distances may wish to consider the possibility of adopting the 75 ohm unbalanced interconnect specified in AES3-ID or SMPTE 276M as an alternative to balanced 110 ohm interfacing. Convertors are available which perform the job very easily. It has been suggested by a number of sources that HF interference such as RF sources is better rejected by the effectiveness of cable screening than by the balance of the electrical interface, and that 75 ohm coax cable has better controlled impedance than audio microphone cable.

A final point to bear in mind is that although transformers are not mandatory in most versions of the two-channel interface standard they may be used to ensure good electrical isolation and earth separation where appropriate. The transformer is standard in the EBU version of the interface, since it was regarded as important in broadcast studio centres where earth continuity is normally avoided between operational areas.

6.7.3 Data Mismatch in Professional Systems

Data mismatch between professional devices using the standard two-channel interface has typically been confined to problems with the implementation of channel status (see section 4.8), but there is also the possibility for differences in sampling rate and audio word length, as discussed below. In more recent systems there are numerous options for higher sampling frequencies, operating the interface in single-channel-double-sampling frequency mode or simply increasing the data rate, which can lead to difficulties in communication. There is also the increased likelihood that interfaces may be operated in the 'non-audio' or 'other purposes' mode for carrying data-reduced audio signals, as described in Chapter 4. User bit incompatibilities might arise if greater use was made of this channel, but few current professional devices take any notice of the state of the user bit. The validity bit is historically another root of trouble and its handling was discussed in section 4.6. As discussed in section 6.12, a number of manufacturers now produce devices specifically designed to analyse and/or correct for data incompatibilities between pieces of equipment and such 'fix-it' boxes can be very useful in encouraging communications between systems.

Incompatibilities in channel status implementation can give rise to a variety of symptoms ranging from complete failure of communication to seemingly correct but actually improper communication. The reason that such incompatibilities have arisen is largely that the original AES3 specification was less than specific on how devices should set channel status bits that were not used, as well as how they should respond when receiving data that they were incapable of handling. Consequently all sorts of channel status implementations exist in commercial products, although it should be said that most of the time devices communicate without problems. Because of these potential difficulties, AES3-1992 was more specific about channel status implementation, specifying three levels of implementation depending upon the application (see section 4.8.4). As a result there should be less difficulty between modern devices than between older ones.

The most common problem areas in channel status implementation were historically (a) in the CRCC byte (byte 23); (b) in the signalling of pre-emphasis; (c) in the setting of the consumer/professional flag; and (d) in the indication of sampling rate. Less common problems arose when the left and right channels had different channel status data (making it difficult to decide which was correct) or where the channel status block was the wrong length (one famous example exists of a 191-byte channel status block!).

The problem with the CRCC byte was that not all transmitting devices included it at the end of the channel status block and some early devices implemented it incorrectly, thereby confusing those that expected to see the correct CRCC data. Receivers that check the CRCC will indicate an almost continuous CRC error when decoding an input signal that does not contain CRCC or where it is incorrectly implemented. The reaction of such a receiver will vary from complete refusal to accept the signal to acceptance of the signal whilst flagging a CRC error. It is impossible to state what the 'correct' response should be in such a case, as there is no way of telling whether the channel status data is correct or not. Interface signal processors such as the 'fix-it' boxes mentioned above often perform the useful function of inserting correct CRCC data into the channel status blocks of signals that lack it. It might reasonably be assumed that a device seeing CRCC bytes repeatedly set to zero would assume that it was not in use and thereafter ignore it, but this is rarely the case at present.

The type of pre-emphasis used should be indicated in bits 24 of byte 0 of channel status and it is possible that emphasis may have been applied to a signal without indicating this in channel status. Such incorrectness will not prevent the interface from working but may give rise to a pre-emphasized signal being carried through further stages in the signal chain without being de-emphasized. The only way to tell if a signal is pre-emphasized (when it is not indicated) is really to listen to it, since pre-emphasized signals will have an exaggerated HF response. The correct pre-emphasis flags may be set using a suitable interface processor, or the signal may be de- emphasized in the digital domain and the flags set to the no emphasis state.

Apart from the consumer/professional flag (discussed below), the sampling frequency indication is the other main area of difficulty in channel status. Not all devices indicate the sampling frequency of the signal in bits 6 and 7 of byte 0, and this can cause a lack of communication when received by a device expecting to see such data. Japanese devices in particular often will not accept data if it does not have the sampling frequency flags set correctly. There is also now the additional indication of sampling frequency in byte 4 of channel status (AES3, Amendment 3 1999) to complicate matters, although this is not a requirement for correct functioning of the interface. It is difficult for a receiver to know what to do when presented with a sampling rate flag that contradicts the true audio sampling rate. In such cases it might be suggested that the device should rely on its detection of the true rate rather than the indicated rate, whilst perhaps flagging a mismatch on the front panel. Some confusion also exists over the interpretation of bit 5, byte 0, which indicates 'source sampling frequency unlocked'. The question is 'with reference to what is the sampling frequency unlocked?' the internal clock, an external reference? When this bit is set to '0' (the default state) nothing can be concluded about the locked state of the sample clock and many systems do not set this bit anyway. When set to '1' all one can say is that there is some problem with the lock of the sample clock and that its frequency may not be relied upon. In systems synchronized to a master reference signal its presence could be used to indicate that there was a free-running clock in a source device earlier in the signal chain.

6.7.4 Electrical Mismatch between Consumer and Professional Systems

As described in Chapter 4 there is so much similarity between the consumer and professional interfaces that it is tempting to think that consumer devices can be connected directly to professional systems or vice versa. Indeed there is a strong operational motive for this because consumer and professional digital equipment are often used together in studios and programme material is often copied between systems. The problem is that although it is possible in some cases to make the electrical interconnection work, there are other difficulties to contend with such as the almost total dissimilarity in channel status and user bits. There is also the possibility that the consumer device may have a much less stable sample clock and be unable to lock to an external reference, giving rise either to problems in decoding or the danger that sample clock jitter will be passed on to other devices in the professional system. Ideally, therefore, one should not attempt the direct interconnection of consumer and professional equipment, preferring rather to use one of the many interface convertors that exist on the market which will set the necessary channel status bits correctly and convert the signal to the new electrical format. It may also be necessary to resynchronize the signal from a consumer device using a buffer store or sample rate convertor (see section 6.5) by locking it to the reference signal of the professional system, thereby ensuring that its sampling rate is the same as that of the professional system and hopefully removing any excessive clock jitter.

For cases in which the only solution is to attempt direct interconnections some guidelines will be given. Clearly such set-ups should be viewed as temporary and allow for the possible incompatibilities in channel status. Consumer-to-professional electrical connection is often possible because the consumer interface peak output voltage is around 0.5 V and the minimum allowed input voltage to a professional system is 0.2 V. Provided that the wire is not too long the signal may be decoded. As shown in Figure 6.12 the centre core of the consumer coaxial lead may be connected to pin 2 of the professional XLR and the shield to pins 1 and 3. Clearly there will be an impedance mismatch and commercial impedance transformers are available to convert between either 75 and 250 ohms or 75 and 110 ohms. One Japanese manufacturer recommends the circuit shown in Figure 6.13 to balance a consumer output for carrying it over longer distances.

Figure 6.12: Temporary method of interconnection between consumer and professional interfaces.

Figure 6.13: An example of a circuit suggested by one manufacturer for deriving a balanced digital output from consumer equipment.

Professional-to-consumer connection may also work, again depending on channel status, since a consumer input is not normally damaged by the higher professional signal voltage. Two circuits are shown in Figure 6.14, depending on whether the professional output is transformer balanced (floating) or driven directly from TTL-level chips (balanced, but not floating). It is also possible to convert signals between consumer electrical and optical formats. Suggested circuits are shown in Figure 6.15.

Figure 6.14: Examples of impedance matching adaption circuits between professional and consumer devices (a) non-floating, and (b) floating (transformer-balanced) professional sources.

Figure 6.15: An example of a circuit suggested by one manufacturer for converting (a) consumer electrical to consumer optical format, and (b) optical to electrical.

Unfortunately the original IEC 958 did not exclude the possibility of using the Type 2 electrical interface with professional data , or indeed vice versa (the two subjects were entirely separate in the document). This occasionally led to some unexpected implementations. Although one normally expects to find professional data coming out of XLR connectors there are cases where manufacturers or dealers have simply taken a consumer device and provided it with a so-called 'professional' output by feeding the consumer output via an RS-422 driver to an XLR connector. This may be cheap but it is to be discouraged since it leads users to think that the equipment may be connected directly to professional systems, whereas in practice the professional system may refuse to accept it due to the consumer flag being set in the first bit of channel status. If the data is accepted it is possible that further difficulty may arise due to the misinterpretation of channel status data.

6.7.5 Data Mismatch between Consumer and Professional Systems

Although the audio part of the subframe is to all intents and purposes identical between the two interfaces there are key differences in the channel status data that have already been described in theory in Chapter 4. The success or otherwise of directly interconnecting consumer and professional equipment depends to a large extent on how this data is interpreted.

Some professional systems are provided with both consumer and professional interfaces and clearly this offers the ideal solution, but some older equipment had only one electrical interface with a switch to select 'consumer' or 'professional' data characteristics. Often the 'consumer' implementation simply meant that it would accept data with the first bit of channel status set to zero, ignoring most or all of the following data. The similarity in channel status between consumer and professional runs out fairly quickly after the first couple of bits. One of the most common problems is in the signalling of pre-emphasis. When a professional signal with 'emphasis not indicated' is received on a consumer device without any intervention, it will assume that the copy protection flag is being asserted and prevent the signal being recorded (depending on SCMS, as described in section 4.8.7). Similarly, a copy-protected consumer recording being copied directly to a professional system without intervention would force the professional device to a 'no emphasis' state, although a non- copy-protected consumer recording would normally be OK and would set the emphasis state correctly.

Past the emphasis bits there is no similarity at all between the interfaces, and thus it is difficult to say exactly what the results of one system interpreting the other's channel status data would be. Ideally a receiver should check the first bit of channel status to detect the consumer or professional nature of the data, and then switch its implementation automatically to interpret it accordingly .