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Understanding Consumer Digital Audio Formats

Last time we looked at the standards for the professional digital audio interface, AES3-2003 and SMPTE276M, as well as an AES information document, AES-3id-2001 (r2006). AES3 describes a balanced interface using 110 ohm shielded twisted pair ohm audio cable, while SMPTE276M and AES-3id specify an unbalanced interface using 75 ohm coax.

There’s also a consumer digital audio format that uses an unbalanced interface, but it differs in several important aspects from SMPTE276M or AES-3id. The consumer format is commonly called S/PDIF (Sony/Philips Digital Interface Format) and is standardized in IEC60958-3.

Electrically, the S/PDIF interface is unbalanced with an impedance of 75 ohms and a nominal voltage of 0.5 V peak-to-peak. RCA connectors are used for digital inputs and outputs. The cable could be coax, but on very short interconnects, it may be possible to use unbalanced audio cable. S/PDIF can also appear on optical connections as well.

The overall data structure for the consumer digital audio format is like the AES3 signal, in that it contains a preamble, digital audio sample word, and bits for validity, user data, channel status, and parity. The coding of the digital audio itself is the same as AES3. The differences come about in the user bits and more importantly in the channel status bits. These differences are the reasons that S/PDIF and AES3 or AES-3id are often incompatible.

In the consumer format, the user bits can contain data relating to the audio program, and the specification describes how different classes of equipment should handle it. In AES3, by comparison, there’s more flexibility in how the user bits are employed. For example, user bits in AES3 can be user defined, be in the form of a 192-bit block like the channel status bits, or be the same as the consumer format, depending on the particular setting of the channel status bits assigned for user bit management.

Remember that one AES3 frame consists of two subframes, one for each channel. In the AES3 signal there are two streams of user bits, one for subframe 1, which is associated with the audio channel carried in that subframe, and one for subframe 2. But for the consumer format, user bits for subframe 1 and subframe 2 are combined into one common stream.

(click thumbnail)Table 1: Comparison of channel status bits for consumer and professional digital audio signal. References: AES3-2003; “Audio Precision Application Note #5, Measurement Techniques for Digital Audio” by Julian DunnBIT SETTINGS

Now let’s look at the channel status bits. Bit 0 is the key. When Bit 0 of the channel status bits is set to 0, this indicates consumer format. If Bit 0 is set to 1, that means professional format. But that’s not all. Depending on how Bit 0 is set determines the functions of the rest of the bits used in channel status.

Table 1 lists the functions of the various channel status bits for both consumer and professional formats. As can be seen, Bits 2 and higher don’t correspond.

In comparing S/PDIF and AES3, keep in mind that there are electrical incompatibilities between the two as well as signal incompatibilities. Table 2 compares the electrical characteristics for each of the digital audio variations discussed. For the signal, it’s important to consider not only the digital audio samples but also the sub-codes like user bits and channel status. In short, S/PDIF and AES3 are not the same. AES-3id and S/PDIF are not the same, even though both are unbalanced 75 ohm interfaces.

And yet, confusingly so, often a S/PDIF output can be connected to an AES-3id input (via a RCA to BNC adapter) and work. It just depends on how picky the receive device is about the user and channel status bits, and if the receive device has an automatic sample rate converter. While the standard for the consumer format allows for sampling rates of 48 kHz and 32 kHz, the sampling rate for most typical consumer devices is 44.1 kHz, while for broadcast applications it’s 48 kHz. (AES3 also allows sampling rates of 44.1 kHz and 32 kHz. The appropriate channel status bits indicate which is used.)

Usually, we’re not as lucky going directly from an AES3 or AES-3id output to a S/PDIF input, since many consumer devices want to see that consumer bit set. Also sample rate conversion may need to be performed.

(click thumbnail)Table 2: Table 2: Summary of the differences of key electrical parameters of professional and consumer digital audio interfaces. References: AES3-2003; AES-3id-2001 (r2006); SMPTE276M; “RaneNote 149—Interfacing AES3 & S/PDIF” by Dennis BohnSYSTEM DESIGN

So, when designing a digital audio system with both pro AES3 and consumer S/PDIF digital audio signals, here are some things to consider.

The most reliable way of interfacing S/PDIF devices to AES3 devices is through a format converter.

A format converter will, at a minimum, change the appropriate sub-code bits in the S/PDIF signal to convert it into AES3 or AES-3id. Conversely, this type of device can also change the AES3 or AES-3id format to S/PDIF. A format converter should also produce the correct voltages that are nominal for each format, namely 4 volts for AES3, 1 volt for AES-3id, and 0.5 volts for S/PDIF. Examine the specs carefully for converters you are considering.

One disadvantage of format converters is the additional expense, plus the extra rack space and power strip outlets that are needed to accommodate them. If a system contains many S/PDIF devices, the dollars and rack space can quickly add up.

If the S/PDIF devices primarily feed an audio console, check with the console manufacturer to see if the console’s inputs can accept such a signal, and if there is automatic sample rate converters on each channel. That could obviate the need for about half of the converters. Still it’s best to test with the actual equipment you’ll be using.

If you know that not every S/PDIF device will be used at the same time, another approach is to provide a pool of format converters and possibly sample rate converters that can be patched in or routed as needed, rather than dedicating one or two per device. Wire all the S/PDIF devices to patchbays dedicated just to S/PDIF, and AES3 devices on their own set of patchbays. Label each clearly as to signal type. Install utility converters with their inputs and outputs on the appropriate patchbays. That way you will be able to patch S/PDIF and AES3 devices together through format and sample rate converters as needed.

Next time we’ll look at the options for interfacing between AES3 and AES-3id.