What is SCMS? - Detailed

This is an SCMS primer for the layman. It is not a full technical description, but should give even the novice, with no technical knowledge, a basic understanding of the system. The digital audio interface signal format used for all consumer units is S/PDIF. This signal is transmitted as an unbalanced data stream. Transmission methods used are either standard coaxial or optical. There are two copy managment systems that are built into DAT machines.

The first is called the Social Gathering System (SGS) and was used prior to May 1990. It is implemented only in early DAT machines like the Sony DTC 1000ES. It used ID6, just like the newer SCMS system, however, the 11 status was designated as "reserved for future use" at that time. Early models will, therefore react to ID6 status 11 as copy prohibited data.

The second is the now dreaded SCMS system, which places copy prohibit bits on data that was never Copyrighted, in what seems like an attempt to bleed the working man of even more money to support the well paid Record Industry Executives. We feel this should only have been enforced with originally Copyrighted data. This would be fair to all. As it is the SCMS system has done nothing to stop the stealing of Copyrighted music. Does anyone think a thief is concerned with the fact that the music he is selling has gone thru the A to D then D to A process? I doubt it. The only ones affected by this are the individuals who cannot afford a pro machine or SCMS stripper. SCMS has not prevented a single sale of Copyrighted material (although it has prevented the sale of SCMS machines!). This system was adopted as an article of commerce by the Ministry of International Trade and Industry in May of 1990. I hope they are proud of their achievment (and I hope karma is a fact !).

Let's start with a brief description of the DAT format. The DAT recording format uses two helically scanned tracks. These are used for interleaved "frames" of digital audio data and for sub-code data. PCM audio data is recorded in the main data area which is comprised of 128 blocks. Each block is comprised of 192 frames. Each of these frames contains 32 bits and represents either left or right channel data. These 32 bits sections are called sub-frames which are described later. The main ID section of the data steam is where address information, ID codes and the like are stored. The DAT sub-code information is stored on each track in two eight block groups. Sub-Code areas which store such information as track number, absolute time, track time and the like have the unique ability to be re-recorded at a later time. The PCM audio data cannot be re-recorded without re-recording the sub-code data. The S/PDIF signal contains both left and right channel information. This multiplxed signal requires the creation of the sub-frames, transmitted in blocks of 192 frames , as mentioned earlier. the SCMS system uses the second bit of the channel status information to either permit or prohibit engaging the transport mechanism on consumer, or DAT catagory coded machines. In these machines you will see levels and can hear audio, but the machine will not come out of the pause mode. Catagory DAT-PRO machines ingnore the SCMS prohibit bit.

The following list the assignment of ID1 thru ID6:

ID # | Purpose | Bit Assignment
ID1 | Emphasis | 00=off 01= 50/15
ID2 | Sampling Rate | 00=48k 01=44.1k 10=32k
ID3 | Number of Channels | 00=2ch 01=4ch
ID4 | Quantization | 00=16bit 01=12bit
ID5 | Track Pitch | 00=normal 01=Wide
ID6 | SCMS (digital copy) | 00=yes 10=no 11=once
ID7 | Pack | Pack contents

The sub-frame format is as follows. The pre-amble (bits 0 thru 3) , which are used as a sync signal and sub-frame markers in the downstream machines. This enables the receiving machine to properly reassemble the incomming data. The auxiliary data ( bits 4 thru 7) reserved for use as bits 21 thru bits 24 in 24 bit DAT recorders of the future and used today by Hi End digital recorders and Digital Audio Workstations. The 20 bit allocation area (bits 8 thru 11) reserved for 20 bit DAT recorders. As far as We know all machines hold bits 8 thru 11 low even today. Bits 12 thru 27 contain the 16 bit PCM audio data we listen to on most gear. Bits 28 thru 31 are four control flags labeled as the Validity flag, User data, Channel status and Parity. The Validity flag is used to indicate an error in the audio word contained in the sub-frame. The User data is used to transmit sub-code information from one machine to another. The Channel Status bit, 192 of which are grouped into one block. This channel status foramt block transmits channel A and B features and indentify pro ( catagory DAT-P), consumer (catagory DAT), CD, PCM processor (ie the beloved PCM F1 format ) and Broadcast Satelite data transmission (this is why some pro to consumer or consumer to pro cables can produce tapes that will dub on some machines, but not others). The Parity bit is used to set signal polarity during transmission (why early attempts to invert data to defeat SCMS did not work !).

Copy protect schemes are designed to react to ID6 data bits as THEY COME OFF THE TAPE ! SCMS copy prohibit bits are NOT written into the data stream when data is being transmitted from the A/D convertor nor are they incremented when passed thru a dat machines digital inteface connectors. The system has no need to alter data passed thru its I/O ports. This is why the Data received thru the RX terminal is simply passed to the TX terminal.