Digital Audio Recorders
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Digital Audio Recorders Consumer Digital Tape Recorders The first commercially available digital recorders made use of videotape recorders as the actual storage medium. The PCM (pulse code modulator) converters digitized the audio signal and converted it to a video signal. These recorders were the Sony PCM F-1 and the later PCM 501, 601, and 701 processors. They allowed 14 or 16 bit quantization with better error correction in the 14 bit mode. The main problem with these units was due to poor tracking on the VCR that led to dropouts and adjustment of the finicky VCR tracking control was frequently required. Only the 601 offered digital I/O. PCM converters are rarely used today and are frequently available used for a few hundred dollars. The DAT (digital audio tape) recorder was developed later, incorporating both the A/D and D/A converters and the actual tape transport mechanism into a single unit. The DAT offered better (although not perfect) machine-to-machine matching and greater simplicity of operation. DAT recorders have better error correction and automatic tracking, making them preferable to PCM/video recorders. DATs also offer sub-code indexing not available on PCM converter systems. Most DAT recorders are 16-bit machines, although there were 24-bit variants. Extending the basic technology of the DAT, rotary-head multi-track digital recorders were developed. The Alesis ADAT and the Tascam DTRS recorders used video transports with rotary heads optimized for digital audio to create video-tape-based recorders capable of 8-channel simultaneous recording and playback. These machines are still in use widely and have decided advantages, since they may be linked easily to provide large track counts and may dub with no signal degradation. Tapes may easily be cloned and shared between studios, making collaborative work easy. The ADAT machines managed to ultimately provide 20-bit recording at 48 kHz, while the DTRS format now allows 24-bit recording at up to 48 kHz sample rates and higher rates can be accomplished by “bit-splitting”, effectively doubling the sample rate at the expense of halving the track count. Although rotary-head technology is really an interim solution to the problem of digital recording, it has proven very popular and is likely to continue for some time even as hard-disk-based recording becomes increasingly desirable and inexpensive. More recent recording technologies have made use of perceptual coding to reduce the amount of audio data required for storage, resulting in systems like the MiniDisc, which uses magneto-optical disks that are read optically. MiniDiscs can be re-written, so punch-ins and overdubbing are possible. MiniDisc recorders are available as stereo recorders and as small multi-track systems that are definite improvements on cassette multi-tracks. Since they do employ psychoacoustic principles to reduce the data actually stored, they are not the best choice for master recordings, which may later be altered through equalization or other processing. Recorders using CD-R media are also now available as stand-alone recorders without resorting to data reduction techniques and re-writable CD-R recorders are also beginning to come to market. Professional Digital Tape Recorders: Professional stationary-head, reel-to-reel digital recorders were around for some time, with Sony, Mitsubishi, and other manufacturers making 24, 32, and 48-track machines. Different systems developed: DASH (Digital Audio Stationary Head) and ProDigi competed for users. Ultimately, the DASH format won out, but currently these expensive machines are declining in popularity as much cheaper digital systems like the ADAT and hard- disk systems have become entrenched. They could record up to 48 tracks on 1/2” tape and could be edited manually, but all of this is now possible on cheaper and simpler systems. For example, a used Sony 3348HR 24-bit/48 channel machine sold for well over $100,000. Six DA-78HR machines are under $15,000 and the modular nature of these machines makes maintenance less of a problem. Interesting variants include the Yamaha DMR/DRU series, which used stationary-head technology and proprietary 8mm tape cassettes to record 8 tracks of 20-bit digital audio at up to 48 kHz. The DMR-8 included 24-track moving-fader automation, full SMPTE sync, and 3 built-in digital processors in a professional porta-studio system. Since a full system would cost over $50,000, it was abandoned when the ADAT hit the market. Now, the popularity of tape-based digital recorders has declined further. While they still make sense in some applications, they are most useful in shared facilities where the easy interchange of tapes makes switching from project to project easier or for on-the-road uses like recording live shows or backing-track playback. Computer- based systems are even making inroads into these last applications, as they become more resistant to poor- quality power that may require time-consuming rebooting in the middle of critical recordings or playback. It would appear the days of tape-based digital recording are numbered, as manufacturing of all but the top level professional machines has stopped. These are mainly intended for the post-production market where they were adopted as the standard and are still used. The computer workstation has claimed the music-recording market, which it still shares with analog tape. The most popular tape-based stereo digital recording system is the R-DAT or, as it is more commonly known, the DAT. The R-DAT uses a rotary head to increase the data density recorded much like a video cassette recorder. The rotary head leads to a helical scan in which the rotating head writes tracks at an angle to the direction of tape travel, resulting in a greater effective head-to-tape speed. There are also S-DAT recorders, which use fixed, or stationary, heads and require high tape speeds and multiple tracks /per audio channel to achieve the required data densities. The Yamaha DMR-8 was one of the few S-DATs. The professional digital reel-to-reel recorders used fixed heads and are referred to as DASH (digital audio stationary head) format machines (Mitsubishi’s competing Pro-Digi (PD) format has been withdrawn from the market). Sony has developed a relatively inexpensive 24-track DASH machine, but most currently affordable digital recorders like the Alesis 8-track ADAT and the Tascam DA-88 use rotary head systems, employing modified videotape transports. The development of magnetic and magneto-optic disk technologies has spawned a new generation of disk-based recording systems that has largely supplanted digital tape-based systems, especially for multi-track applications. The disk-based systems allow features impossible on tape-based systems, like non-destructive editing and virtual tracks. They make use of inexpensive, mass-produced hard disks and computer technology and are gaining in popularity. DAT (Digital Audio Tape) DAT recorders record two audio channels and are capable of some degree of data recording as well: absolute time code (although not SMPTE), indexes, and start IDs can be recorded. These data are recorded in what is called the sub-code area. There are also automatic track finding data written on the DAT, allowing precise track alignment between machines. These data can be read even in fast forward or reverse, allowing quick and precise cueing. The DAT recorder combines elements of videotape and CD digital audio technologies. Like videocassette recorders (VCRs), a rotating head increases the data density by writing tracks diagonally on the tape, thus increasing the effective head-tape velocity. This requires servo control of tape speed to maintain the proper alignment of tape track and head gap. The rotating drum contains two separate heads 180 degrees apart. Each head maintains contact with the tape for 90 degrees, with the tape wrapped around the head the same 90 degrees. This results in a discontinuous signal coming off the heads. Since our digital audio signal is discontinuous already, this does not present a problem: the output signal from the head is stored and played back at the appropriate rate. The angles of the two head gaps are different: one at -20 degrees azimuth and one at +20 degrees azimuth. This is done so that adjacent tracks are at a 40 degrees angle with each other, the resulting phase shift greatly reducing crosstalk between the tracks. This eliminates the need for a guard band between tracks, increasing data density. In order to keep the 13.6 micron-wide tracks aligned with the head rotating at 2000 RPM, sophisticated methods are used to synchronize tape movement with the rotation of the heads. On either side of the PCM data, a special ATF (automatic [or area-divided] track following ...not alcohol, tobacco, and firearms...) signal burst is recorded. Each head can use the ATF signal from the opposite head/track to servo the capstan rotation and thereby align the tracks and heads. In addition to audio data and ATF bursts, there are areas of each track devoted to so-called sub-code information. This may include the absolute time code, start and skip IDs, and program numbers. The DAT provides more sub-code data than do CDs. The data generated by the A/D converter is manipulated to add error-detection redundancy, interleaving, and is processed using eight-to-ten modulation before it is sent to the heads. Eight-to-ten modulation uses ten-bit codes to encode the eight-bit data bytes. This eliminates the DC component that would act to magnetize the heads and reduces long wavelength signal components, which makes the azimuth recording system perform more reliably. To reduce dropout damage, each track is split between left and right channel data.