4.12 Manufacturer-Specific Interfaces
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4.12 Manufacturer-Specific Interfaces
Interfaces other than the internationally standardized two-channel and multichannel types will be described and discussed in this section. For example, a number of interfaces have been introduced by specific manufacturers and are normally only found on that manufacturer's products, or are licensed for use by others. Some of these proprietary interfaces have become quite widely used in commercial products for example, the ADAT 'lightpipe' interface is widely encountered on computer sound cards because it is a small- sized optical connector capable of carrying eight channels of digital audio. Some of the technology described in this chapter is the subject of patents and implementers may need to enter into licensing agreements.
MIDI, included briefly in the previous editions of this book, is not strictly a digital audio interface and coverage of it has been removed in this edition. The interested reader is referred to MIDI Systems and Control 35 or the forthcoming Desktop Audio Technology by Francis Rumsey.
4.12.1 Sony Digital Interface for LPCM (SDIF-2)
Sony's original interface for linear PCM data was SDIF-2. It was designed for the transfer of one channel of digital audio information per physical link at a resolution of up to 20 bits (although most devices only make use of 16). The interface has also been used on equipment other than Sony's, for the sake of compatibility, but the use of this interface is declining as the standard two-channel interface becomes more widely used.
The interface is unbalanced and uses 75 ohm coaxial cable terminating in 75 ohm BNC-type connectors, one for each audio channel. TTL-compatible electrical levels (05 V) are used. The audio data is accompanied by a word clock signal on a separate physical link (see Figure 4.29), which is a square wave at the sampling frequency used to synchronize the receiver's sample clock. Sony's multitrack machines use SDIF also, but with a differential electrical interface conforming to RS-422 standards (see section 1.7.2) and using 50 pin D-type multiway connectors, the pinouts of which are shown in Table 4.11. A single BNC connector carries the word clock as before.
| Pin | Function |
|---|---|
| 1, 2 | Ch. 1 (-/+) |
| 3, 4 | Ch. 2 (-/+) |
| 5, 6 | Ch. 3 (-/+) |
| etc. | etc. |
| to | to |
| 47, 48 | Ch. 24 (-/+) |
| 49, 50 | NC |
Figure 4.29: SDIF-2 interconnection for two audio channels.
In each audio sample period, the equivalent of 32 bits of data is transmitted over each physical link, although only the first 29 bits of the word are considered valid, since the last three-bit cell periods are divided into two cells of one-and-a-half times the normal duration, violating the NRZ code in order to act as a synchronizing pattern. As shown in Figure 4.30, 20 bits of audio data are transmitted with the MSB first (although typically only 16 bits are used), followed by nine control or user bits (although the user bits are rarely employed). A block structure is created for the control/ user bits which repeats once every 256 sample periods, signalled using the block sync flag in bit 29 of the first word of the block. The resulting data rate is 1.53 Mb/s at 48 kHz sampling rate and 1.21 Mb/s at 44.1 kHz.
Figure 4.30: At (a) is the clock content of the SDIF signal; (b) shows the synchronizing pattern used for data reception . At (c) user bits form a block that is synchronized every 256 sample periods.
The SDIF-2 interface was originally used mainly for the transfer of audio data between Sony professional digital audio products, particularly the PCM-1610 and 1630 PCM adaptors, but also from semi-professional Sony equipment which had been modified to give digital inputs and outputs (such as the PCM-701 and various DAT machines). It has also been used on a number of disk-based workstations. It is not recommended for use over long distances and it is important that the coaxial leads for each channel and the word clock are kept to the same length otherwise timing errors may arise. Problems occasionally arise with third-party implementations of this interface that do not use the 1.5-bit cell sync pattern at the end of words, requiring some trial and error involving delays of the data signal with relation to the separate word clock in order for the link to function correctly.
4.12.2 Sony Digital Interface for DSD (SDIF-3)
Sony has recently introduced a high-resolution digital audio format known as 'Direct Stream Digital' or DSD. This encodes audio using one-bit sigmadelta conversion at a very high sampling frequency of typically 2.8224 MHz (64 times 44.1 kHz). There are no internationally agreed interfaces for this format of data, but Sony has released some preliminary details of an interface that can be used for the purpose, known as SDIF-3. Some early DSD equipment used a data format known as 'DSD-raw' which was simply a stream of DSD samples in non-return-to-zero (NRZ) form, as shown in Figure 4.31(a).
Figure 4.31: Direct Stream Digital interface data is either transmitted 'raw' as shown at (a) or phase modulated as in the SDIF-3 format shown at (b).
In SDIF-3 data is carried over 75 ohm unbalanced coaxial cables, terminating in BNC connectors. The bit rate is twice the DSD sampling frequency (or 5.6448 Mbit/s at the sampling frequency given above) because phase modulation is used for data transmission as shown in Figure 4.31(b). A separate word clock at 44.1 kHz is used for synchronization purposes. It is also possible to encounter a DSD clock signal connection at 64 times 44.1 kHz (2.8224 MHz).
4.12.3 Sony Multichannel DSD Interface (MAC-DSD)
Sony has also developed a multichannel interface for DSD signals, capable of carrying 24 channels over a single physical link 36 . The transmission method is based on the same technology as used for the Ethernet 100BASE-TX (100 Mbit/s) twisted-pair physical layer (PHY), but it is used in this application to create a point-to-point audio interface. Category 5 cabling is used, as for Ethernet, consisting of eight conductors. Two pairs are used for bi-directional audio data and the other two pairs for clock signals, one in each direction.
Twenty-four channels of DSD audio require a total bit rate of 67.7 Mbit/s, leaving an appreciable spare capacity for additional data. In the MAC-DSD interface this is used for error correction (parity) data, frame header and auxiliary information. Data is formed into frames that can contain Ethernet MAC headers and optional network addresses for compatibility with network systems. Audio data within the frame is formed into 352 32-bit blocks, 24 bits of each being individual channel samples, six of which are parity bits and two of which are auxiliary bits.
In a recent enhancement of this interface, Sony has introduced 'SuperMAC' which is capable of handling either DSD or PCM audio with very low latency (delay), typically less than 50 s. The number of channels carried depends on the sampling frequency. Twenty-four DSD channels can be handled, or 48 PCM channels at 44.1/48 kHz, reducing proportionately as the sampling frequency increases . In conventional PCM mode the interface is transparent to AES3 data including user and channel status information.
4.12.4 Tascam Digital Interface (TDIF)
Tascam's interfaces have become popular owing to the widespread use of the company's DA-88 multitrack recorder and more recent derivatives. The primary TDIF-1 interface uses a 25 pin D-sub connector to carry eight channels of audio information in two directions (in and out of the device), sampling frequency and pre-emphasis information (on separate wires, two for f s and one for emphasis) and a synchronizing signal. The interface is unbalanced and uses CMOS voltage levels. Each data connection carries two channels of audio data, odd channel and MSB first, as shown in Figure 4.32. As can be seen, the audio data can be up to 24 bits long, followed by two bits to signal the word length, one bit to signal emphasis and one for parity. There are also four user bits per channel that are not usually used. This resembles a modified form of the AES3 interface frame format. An accompanying left/right clock signal is high for the odd samples and low for the even samples of the audio data. It is difficult to find information about this interface but the output channel pairs appear to be on pins 14 with the left/right clock on pin 5, while the inputs are on pins 1310 with the left/right clock on pin 9. Pins 7, 1417 (these seem to be related to output signals) and 2225 ( related to the input signals) are grounded. The unbalanced, multi-conductor, non-coaxial nature of this interface makes it only suitable for covering short distances up to 5 metres.
Figure 4.32: Format of TDIF data and LRsync signal.
4.12.5 Alesis Digital Interface
The ADAT multichannel optical digital interface, commonly referred to as the 'light pipe' interface or simply 'ADAT Optical', is a serial, self-clocking, optical interface that carries eight channels of audio information. It is described in US Patent 5,297,181: 'Method and apparatus for providing a digital audio interface protocol'. The interface is capable of carrying up to 24 bits of digital audio data for each channel and the eight channels of data are combined into one serial frame that is transmitted at the sampling frequency. The data is encoded in NRZI format for transmission, with forced ones inserted every five bits (except during the sync pattern) to provide clock content. This can be used to synchronize the sampling clock of a receiving device if required, although some devices require the use of a separate 9 pin ADAT sync cable for synchronization. The sampling frequency is normally limited to 48 kHz with varispeed up to 50.4 kHz and TOSLINK optical connectors are typically employed (Toshiba TOCP172 or equivalent). In order to operate at 96 kHz sampling frequency some implementations use a 'double-speed' mode in which two channels are used to transmit one channel's audio data (naturally halving the number of channels handled by one serial interface). Although 5 metre lengths of optical fibre are the maximum recommended, longer distances may be covered if all the components of the interface are of good quality and clean. Experimentation is required.
As shown in Figure 4.33 the frame consists of an 11-bit sync pattern consisting of 10 zeros followed by a forced one. This is followed by four user bits (not normally used and set to zero), the first forced one, then the first audio channel sample (with forced ones every five bits), the second audio channel sample, and so on.
Figure 4.33: Basic format of ADAT data.
4.12.6 Roland R-Bus
Roland has recently introduced its own proprietary multichannel audio interface that, like TDIF (but not directly compatible with it), uses a 25-way D-type connector to carry eight channels of audio in two directions. Called R-bus it is increasingly used on Roland's digital audio products, and convertor boxes are available to mediate between R-bus and other interface formats. Little technical information about R-bus is available publicly at the time of writing.
4.12.7 Mitsubishi Digital Interfaces
This section is included primarily for historical completeness, as Mitsubishi no longer manufactures digital audio equipment. Mitsubishi's ProDigi format tape machines used a digital interface similar to SDIF but not compatible with it. Separate electrical interconnections were used for each audio channel. Interfaces labelled 'Dub A' and 'Dub B' were 16-channel interfaces found on multitrack machines, handling respectively tracks 116 and 1732. These interfaces terminated in 50-way D-type connectors and utilized differential balanced drivers and receivers. One sample period was divided into 32-bit cells, only the first 16 of which were used for sample data (MSB first), the rest being set to zero. There was no sync pattern within the audio data (such as there is between bits 30 and 32 in the SDIF-2 format). The audio data was accompanied by a separate bit clock (1.536 MHz square wave at 48 kHz sampling rate) and a word clock signal which went low only for the first bit cell of each 32-bit audio data word (unlike SDIF which uses a sampling rate square wave), as shown in Figure 4.34. Status information was passed over two separate channels, which take the same format as an audio channel but carried information about the record status of each of the 32 channels of the ProDigi tape machine. One status channel (Rec 'A') handled tracks 116, and the other (Rec 'B') handled tracks 1732 of a multitrack machine. The pin assignments for these connectors are shown in Table 4.12.
Figure 4.34: Data format of the Mitsubishi multitrack interface.
Mitsubishi interfaces labelled 'Dub C' were two-channel interfaces. These terminated in 25-way D-type connectors and utilized unbalanced drivers and receivers. One sample period was divided into 24-bit cells, only the first 16 or 20 of which are normally used, depending on the resolution of the recording in question. Again audio data is accompanied by a separate bit clock (1.152 MHz square wave at 48 kHz sampling rate) and a word clock signal taking the same form as the multitrack version. No record status information was carried over this interface but an additional 'master clock' was offered at 2.304 MHz. The pin assignments are shown in Table 4.13.
| Pin | Function | Pin | Function |
|---|---|---|---|
| 1, 18 | Ch. 1 (+/-) | 10, 27 | Ch. 10 (+/-) |
| 2, 19 | Ch. 2 (+/-) | 11, 28 | Ch. 11 (+/-) |
| 3, 20 | Ch. 3 (+/-) | 12, 29 | Ch. 12 (+/-) |
| 4, 21 | Ch. 4 (+/-) | 13, 30 | Ch. 13 (+/-) |
| 5, 22 | Ch. 5 (+/-) | 14, 31 | Ch. 14 (+/-) |
| 6, 23 | Ch. 6 (+/-) | 15, 32 | Ch. 15 (+/-) |
| 7, 24 | Ch. 7 (+/-) | 16, 33 | Ch. 16 (+/-) |
| 8, 25 | Ch. 8 (+/-) | 17, 50 | GND |
| 9, 26 | Ch. 9 (+/-) | 34, 35 | Bit clock (+/-) |
| 36, 37 | WCLK (+/-) | 38, 39 | Rec A (+/-) |
| 40, 41 | Rec B (+/-) |
| Pin | Function | Pin | Function |
|---|---|---|---|
| 1, 18 | Ch. 17 (+/-) | 9, 26 | Ch. 25 (+/-) |
| 2, 19 | Ch. 18 (+/-) | 10, 27 | Ch. 26 (+/-) |
| 3, 20 | Ch. 19 (+/-) | 11, 28 | Ch. 27 (+/-) |
| 4, 21 | Ch. 20 (+/-) | 12, 29 | Ch. 28 (+/-) |
| 5, 22 | Ch. 21 (+/-) | 13, 30 | Ch. 29 (+/-) |
| 6, 23 | Ch. 22 (+/-) | 14, 31 | Ch. 30 (+/-) |
| 7, 24 | Ch. 23 (+/-) | 15, 32 | Ch. 31 (+/-) |
| 8, 25 | Ch. 24 (+/-) | 16, 33 | Ch. 32 (+/-) |
| 17, 50 | GND |
| Pin | Function |
|---|---|
| 1, 14 | Left (+/-) |
| 2, 15 | Right (+/-) |
| 5, 18 | Bit clock (+/-) |
| 6, 19 | WCLK (+/-) |
| 7, 20 | Master clock (+/-) |
| 12, 25 | GND |
4.12.8 Sony to Mitsubishi Conversion
Comparing the SDIF format with the Mitsubishi multichannel format it may be appreciated that to interconnect the two would only require minor modifications to the signals. Both have a 32-bit structure, MSB first, with only 16 bits being used for audio resolution, the difference being the sync pattern in Sony's bits 2932, plus the fact that the Mitsubishi does not include control and user bits. The word clock of the Sony is a square wave at the sampling frequency, whereas that of Mitsubishi only goes low for one bit period, but simple logic could convert from one to the other. If transferring from Sony to Mitsubishi it would be necessary also to derive a bit clock, and this could be multiplied up from the word clock using a suitable phase-locked loop. Commercial interfaces are available which perform this task neatly.
4.12.9 Yamaha Interface
Yamaha digital audio equipment is often equipped with a 'cascade' connector to allow a number of devices to be operated in cascade, such that the two-channel mix outputs of a mixer, for example, may be fed into a further mixer to be combined with another mixed group of channels. The so-called Y1 format is monophonic and the Y2 format carries two channels in serial form. The two types are very similar, Y1 simply ignoring the second channel of data.
The two-channel cascade interface terminates in an eight pin DIN-type connector, as shown in Figure 4.35, and carries two channels of 24-bit audio data over an RS-422-standard differential line. The two channels of data are multiplexed over a single serial link, with a 32-bit word of left channel data followed by a 32-bit word of right channel data (the 24 bits of audio are sent LSB first, followed by eight zeros). The word clock alternates between low state for the left channel and high state for the right channel, as shown in Figure 4.36. Coils of 20
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