Digital Audio Recorders

Home , Backing track, Digital audio tape, Digital recording, Multitrack recording, ProDigi, Recording format

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. The data is further interleaved into odd and even data blocks, one for each head. This helps reduce head-related burst errors, since each head scans half the data. A double Reed-Solomon code is employed for error detection and correction.

DASH machines

The DASH format uses reel-to-reel transports and stationary heads to record digital audio. This format includes several versions that vary according to the tape speed, tape width, and the number of audio channels recorded. The DASH format can handle up to 48 channels on 1/2” tape, but 24 channels is the most common format. Because the digital heads cannot be made to record and play at the same time, the problems associated with overdubbing are addressed by using a second write head after the read head and using digital delay to synchronize the playback with the incoming audio.

One advantage of the DASH format over the rotary head systems is the ability to do razor-blade editing: the tape can physically be cut and pasted like analog tape. To allow this, electronic circuitry is employed to conceal the damaged data block and buffer and cross-fade the two ends of the edit. The sophistication of the DASH machines is reflected in their price.

Like the R-DAT systems, the DASH recording format makes use of cross interleaving of data and CRCC error detection, but the details of the systems are different. Also, the DASH systems uses a modulation format called HDM- 1 (high-density modulation-1) rather than the PCM modulation used in R-DAT systems. HDM-1 allows higher data densities than are possible with PCM.

Modular digital multi-tracks (MDMs) An adaptation combining DAT and videotape technologies generated the modular digital multi-track recorder, the ADAT by Alesis and the DTRS from Tascam. Both were eight-track, 16-bit digital audio recorders employing videotape transports. The ADAT is based on S-VHS technology while the DTRS is based on Hi- 8 8mm video technology. The ADAT allows about 40 minutes on a 2 hour S-VHS tape and employs a linear sync track. The DTRS system records 108 minutes on a 2-hour cassette and uses a track finding servo system like the DAT. The machines can use their internal time code to link multiple machines into larger multi-track configurations without external hardware. Both systems can sync to external hardware using SMPTE time code and can dump tracks to other machines through digital connections. The machines can provide complete punch-in overdubbing and seamless digital cross-fades. Individual tracks can be offset in time before D/A conversion, a useful tool for tightening up performances and generating delays. Once again, we are presented with a potential format war: each system has strengths and weaknesses. It would seem that the ADAT is better suited to the home studio (especially since it used to be cheaper) and the Tascam is frequently selected by post- production facilities (it locates faster). Both systems are being licensed to other manufacturers.

Both Tascam and Alesis have continued development on their respective systems. ADAT machines currently deliver 20-bit resolution. Tascam has managed 24-bit recording on the DTRS system with the DA-78HR. Although the ADAT has sold more units, both formats seem to be viable for the near future, although their limited sample rates may signal the end of their popularity.

Yamaha DMR/DRU-8

The Yamaha DMR/DRU-8 machines record eight 20-bit tracks on 8mm tape. Unlike the DAT and Alesis/ Tascam digital multi-tracks, the Yamaha system uses stationary heads. This requires the tape speed be high, so conventional 8mm video tapes cannot be used: Yamaha makes their own longer tapes. The details of the system are proprietary, but several digital tracks are supposed to be used for each audio channel. Like the DASH system, punch-in/punch-out and overdubbing are provided with cross-fading between new and old data. When first released, the DMR/DRU machines were the highest dynamic range multi-track recorders available, recording 20-bit words to tape. These machines use external A/D units to permit upgrading without buying a whole new recorder. Currently, the A/Ds are 19-bit.

In addition to digital recording, the DMR-8 provides moving-fader automated digital mixing and effects processing in one unit. Two external 8-channel digital recorders can be linked and controlled from the DMR-8, allowing 24 track digital recording, or the A/D converters can be used to input analog outputs from synthesizers or analog recorders to allow 24 track automated mixes from a variety of sources. [The DMR/DRU system has been abandoned by Yamaha as of 1995: although its performance is excellent, it couldn’t compete with the less expensive units from Alesis and Tascam.] Both of the rotary-head systems have also now been discontinued except for the Tascam DA-98HR as of 2006. Many are still in use and machines are found on the used market for very reasonable prices as service and parts are still available for the moment.

Hard disk recorders

With the rapidly decreasing price of hard disks, we have seen an explosion in disk-based recorders, both as computer-driven and as stand-alone systems. The main advantages of these systems over tape-based systems involve their random-access and non-destructive editing capabilities. Their major limitation is that when the disk is full, it must be backed up to allow more sound to be recorded, a process which may take up to an hour. While these systems began mainly as stereo editors and off-line mixers, today they can do multitrack recording with overdubbing and can mix (mostly) in real-time. Three of these systems were the Digidesign Sound Designer and ProTools, Sonic Solutions SonicStation, and Studer-Editech Dyaxis II (all but ProTools now out of production), all Macintosh based. They use the Macintosh mainly as a graphical user interface and the signal computations are done on dedicated DSP co-processors. In addition to mixing, some of these systems can do digital equalization and amplitude compression/expansion. Another unique application available on some systems is the ability to selectively remove noise and distortion from recordings by means of spectral analysis.

Computer-based systems were also available for the PC: Event Electronics Layla, Ensoniq Paris, Korg 1212 I/O, and Mark of the Unicorn 2408 interfaces could interface with various sound recording and editing programs to provide multi-track recording on the PC as well as the Mac using the PCI bus. With the spread of USB and FireWire ports on computers, it has become quite simple to add A/D/A converters to a computer, enabling powerful software recorders to take over most of the jobs formerly done with dedicated hardware. Programs like ProTools, Digital Performer, Logic, Cubase and others allow the recording, mixing, and effects processing all to be done within the computer itself, with add-in software plug-ins to perform the functions of dynamic-range processing, equalization, delay-based effects, and even CD burning. In a sense the whole studio has migrated into the PC, making access to recording technology available to the general population. With the currently available cheap and powerful CPUs, computers using fast FireWire or USB2 multi-channel audio interfaces and huge hard disks are rapidly becoming the standard studio recording and mixing hardware. Dedicated DSP processing is no longer necessary, although many systems add such processing to greatly expand the plug-in software capabilities of computers.

Stand-alone disk-based recording systems are available from Yamaha, Korg, Akai, Roland, Tascam, Mackie, and iZ Technology. This type of system emulates the operation of a tape recorder, although they allow cut-and- paste editing, they do not allow mixing. Systems like the Korg D8, Fostex DMT-8, Akai DPS-12 and Roland VS-series combine multi-track recording with digital mixing. Similar systems using minidisk for recording are also available from Yamaha and Tascam among other companies.