4.6 The Validity (V) Bit

The application and value of the validity bit in each subframe were debated widely during the formulation of standards. Originally the V bit was designed to indicate whether the audio sample in that subframe was 'valid' or 'reliable', 'secure and error free' in other words, to show either whether it contained valid audio (rather than something else, or nothing), or if it was in error. It was set to '0' if the sample was reliable, and '1' if unreliable (so really it is an 'invalidity' flag). Since it is only a single bit, there is no opportunity for signalling the extent or severity of the error. What has never been clear is what devices should do in the case of an invalid sample and this is largely left up to the manufacturer. The most common use for the V bit is to signal errors that occurred in the transmitting device, such as when an uncorrectable error is encountered when replaying a recording, for example. But not all devices treat this in the same way, since some signal any offtape CRC error whether it was corrected or not, whereas others only set the V bit if the error was uncorrectable, resulting in interpolation or even muting in the convertors of most systems (this seems the more appropriate solution). But this is not the only use. Interactive CD (CD-I) players, for example, use the V bit to indicate that the audio data part of the subframe has been replaced by non-audio data. This is because otherwise there would be a potential delay of up to 1 block (192 frames ) before the receiving device realized that the channel use had changed from audio to non-audio (this is normally signalled in channel status that is only updated once per block).

In AES3-1992 the description of this bit was modified and now indicates whether the audio information in the subframe is 'suitable for conversion to an analog signal', which may or may not amount to the same thing as before. (A binary '1' still represents the error state.) For example, the V bit may be set if an uncorrectable error arises on a DAT machine which would normally result in error concealment by interpolation rather than muting at the output. Any subsequent device receiving audio data from this machine over the digital interface would see the V bit set true and, if interpreting it literally, would assume that the audio was unsuitable for conversion and mute its output, yet the user might still want to hear it, assuming that the interpolation sounds better than a mute! Receivers vary in this respect, and there are some that always mute on seeing the V bit set true. A further and potentially more serious problem, highlighted by Finger ² , is that a recording device will usually store incoming audio data but has no means of storing the validity flag. In such a case the replayed audio would then be transmitted with no indication of invalidity, even if recorded errors existed.

Another problem arises in devices which simply process the data signal, such as the sample rate convertors or interface processors described in Chapter 6. Should such devices take any action in the case of invalid samples, or should they simply pass the data through untouched? One such device takes the approach of carrying through the V bit state and holding the last sample value in the case of an error, but the solution is less clear when two digital signals are to be mixed together, one which is erroneous and the other not. In such a case it is difficult to decide whether the single mixed data stream should be valid or invalid and there are no clear guidelines on the matter (indeed there is no 'catch-all' solution to such a problem).

In truth, the most appropriate actions to and uses of the V bit are application and product dependent. AES2-ID discusses some of these issues and concludes that it is largely the responsibility of the manufacturer to determine the most appropriate treatment of the V bit in accordance with its general definition. Since it is now recommended that all products implementing AES3 are provided with an implementation table, notes on the treatment of the V bit should be incorporated in a similar place in the relevant manual. It is not recommended that the V bit be used as a permanent alternative to the 'audio/non-audio' bit in the channel status block, as an indicator of ongoing non-audio data within the subframe, but this could be a temporary solution until the start of the next channel status block as noted above. IEC 60958 describes this as a good temporary use of the V bit for consumer applications involving non-audio data.