Sony Corporation developed what is commonly considerd to be the first "fully professional" series of digital audio processors designed to be used with conventional U-Matic and one-inch video transports, which record digital data as an encoded NTSC-format video signal. To enable PCM-1610 and PCM-1630 units to communicate with one another digitally, Sony Corp. developed a proprietary format, referred to as SDIF-2 ("Sony Digital InterFace"). SDIF-2 is featured on a variety of digital recorders, workstations, and processing hardware.
SDIF-2 comprises three ports (usually BNC video connectors) that carry digital bitstreams for Channel #1, Channel #2 and a squarewave word clock at the sampling frequency. The 1610/30 clock, incidentally, is derived from the 60 Hz NTSC monochrome TV standard, to provide 44.1 kHz sample rate for CDs. As can be seen from Fig. 19, each frame of data, transmitted as serial NRZ (non-retum-to-zero), MSB first, comprises 31 bits of audio and sync information, with the companion word clock defining one data cycle. Of the total 32-bit slot or digital word, two trailing bits form a synchronization pattern, while the remaining 29 bits carry audio and auxiliary data.
Figure 19 - Three connectors comprise the SDIF-2 interface, which carry digital bitstreams for Channel #1 and Channel #2 plus word clock.
Bits 1 through 20 comprise the PCM digital audio data (for normal 16-bit signals, the last four unused bits are set to zero), followed by eight control bits (or user bits), and a single block flag bit. Bits 26 and 27 are the emphasis identification bits (signifying 50 
s and 15 s poles), while bit 28 is a dubbing prohibition bit which, when set, disables record mode in the slave device.
The user data is sent in blocks of 256, individual 8-bit words, the block flag bit being set to "1" at the start of each 256-word block, to denote a block sync pulse. In addition, the eight auxiliary bits at the beginning of each block are reserved for the control bits previously mentioned, while the remaining 255 sets within the block are given over to user-definable information.
For two-channel applications, such as between pairs of 1610/1630 processors, or into and out of stereo workstations, for example, the data is normally carried as single-ended, TTL-compatible signals via 75-ohm coaxial cable at sampling frequencies of 44.1 or 44.056 kHz. Sony Corporation has also implemented the interface in other hardware, including its family of DASH-Format PCM-3324/3348 multitrack reel-to-reel transports. In these cases, however, the data are transmitted as electronically balanced/differential, RS422-compatible signals via multiway D-Sub connectors, while a single BNC connector carries word clock. The multitrack machines also operate, in addition to 44.1/44.056, at a sampling frequency of 48 kHz.
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