A designated portion of digital audio data transmitted over IEC958 (AES3, S/PDIF, etc.) between hardware devices. There are a number of bits that get transmitted with each sample that do not contain audio data. The Channel Status Bit is the second to last bit of each digital word. They are accumulated one bit at a time, sample by sample (one bit per sample word) at the receiving device until 192 such bits have been received. This accumulation of bits make up what is known as the Channel Status Block. The Channel Status Block carries all kinds of information that is used to further specify the nature of the digital audio data being transmitted, which includes things like sample rate, emphasis, bit depth, SCMS info, CRC error correction data, and a whole bunch of other minutiae.
Even the meaning of the various Channel Status Bits themselves can be changed by other Channel Status Bits. Most notable of these is the very first bit of the aforementioned 192 bits that make up the channel status block. The ‘PRO’ status bit, as it is known, determines whether the data transmission (specifically the rest of the channel status bits) conforms to the professional standard (a.k.a. AES3, IEC958 Part4) or the consumer standard (IEC958 Part 3, S/PDIF). These changes are somewhat subtle (we’re talking about a few bits of data every 192 samples), but can sometimes cause incompatibilities between some equipment. Most equipment characterized as “consumer” conforms its digital data transfer to the consumer standard (with the appropriate bits set accordingly), while most pro equipment will either conform to the pro standard or be able to switch between the two. Some devices can be incompatible with one or the other data format, though this is pretty rare in practice.