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Fostex RD8

Digital Multitrack Recorder By David Mellor
Published November 1994

The Fostex RD8 digital multitrack combines the Alesis ADAT tape format with a wealth of on‑board synchronisation and control facilities. David Mellor investigates.

While Tascam's response to Alesis's ADAT was to develop their own Hi‑8 8‑track digital format, Fostex, for whatever technically or politically motivated reasons, adopted the Alesis S‑VHS ADAT format. My first concern was to verify that the Fostex RD8 was fully compatible with the Alesis ADAT format, though the fact that the transports and much of the electronics for both machines are built in the same Alesis factory should go some way towards ensuring this. But there is more to this than simple record and playback compatibility, since ideally the two machines ought to be functionally compatible too. I can confirm that you can use a Fostex RD8 with an Alesis ADAT as if they were two Alesis machines. The Fostex RD8 can also be controlled by the BRC, and any data that the RD8 writes onto the tape can be read by a BRC and vice versa.


Fostex's recent advertising campaign asks the question, 'Why pay more for a Fostex ADAT?' The Fostex RD8 may be an Alesis ADAT in disguise, but it is much more than a piece of badge engineering. In fact you won't go too far wrong if you think of the RD8 as an Alesis ADAT plus many of the functions of the BRC in one box.

Using the Alesis ADAT is straightforward when you don't need it to interface with other equipment, but once you start piling up the optional accessories, things get more complicated. Many of these options and facilities are supplied as standard with the RD8, which means that the front panel operation is correspondingly more complex. The key to understanding the RD8 is first to know what it can do.

When timecode is involved, the Alesis ADAT (without the BRC) has to sacrifice a track for time code unless you use a third‑party box to convert the Alesis ADAT's internal timecode to MIDI timecode. The RD8 uses a separate subcode 'track' for timecode, and this data is converted to the required timecode standard on playback, which means that you don't have to physically stripe the tape before using it — though, as with ADAT, you should format the tape before using it for the first time. It's worth mentioning that since digital audio consists of a certain number of samples per second, the timecode must be recorded in such a way that 25 frames (or 30, or whatever) last exactly the same length of time as 44100 or 48000 samples — one second, in fact. If you treat your digital recorder as an analogue input and output device then a lack of exact sync will not be a problem, but if you are using digital inputs and outputs (via the Alesis AI‑1 digital interface), then nasty glitching is virtually bound to ensue sooner or later as the two clocks drift apart. The Fostex RD8 contains a timecode generator which will synchronise with the digital data stream and deal with this potential problem. The generator can be set to internal or external references, so you can easily stripe a tape in isolation or jam to code from a video (assuming that it is recorded in sync with the video frame rate!).

A Willing Slave?

Obviously, as a fully ADAT‑compatible machine, the RD8 provides ADAT sync, and can be the slave to, or master of, an Alesis ADAT. Alternatively, you may want your RD8 to run at exactly the same speed as your video source, in which case you can select a video input as the clock source. In a professional digital audio environment, it is essential to be able to synchronise to a digital audio data stream other than through the digital audio input (via the Alesis AI‑1). Simple digital recorders can't do this, and although you won't have many problems if you only have a couple of digital machines, when you start to increase the complexity of your system you certainly will. The RD8 has a separate word clock input so that you can be sure it will run exactly in step with your other digital equipment. When it comes to timecode, the RD8 will chase and lock to LTC (longitudinal timecode) and will also sync to VITC (vertical interval timecode) without you having to shell out for a convertor. VITC offers the significant advantage that it is readable when the video machine is in still frame, and therefore can be relied upon for accuracy. The RD8 does not, however, respond to MTC, though it does respond to MIDI Machine Control. If you need the RD8 to respond to transport commands provided from another machine, you will either have to use its Sony‑compatible 9‑pin socket, or use it in conjunction with MIDI Machine Control compatible software. An alternative means of improving upon simple chase sync is to use the high‑speed timecode‑reading facilities of the RD8 with a master machine capable of outputting timecode in fast wind.


I don't feel it was a good idea to replace the Alesis ADAT's jack sockets with phonos; neither was it a good idea to replace the Alesis' 'pro' level EDACs with 25‑pin D‑connectors; for those who need to move equipment from place to place, fiddly, hard‑to‑wire multipin connectors are not going to help. At least the timecode connections are on proper XLRs.

One thing that isn't quite as clear as it should be is the arrangement of the Record Ready switches. Maybe it's a small point, but it was concern over looks rather than practicality that made the Format and Generator Set‑up buttons identical and placed them in the same row as the Record Readies. Format and Generator are buttons that you use once each per tape, while the Record Readies may be used many times, and should be so easy to use that you hardly have to glance at them. Here you must look carefully, or at least straight on, at the machine to make sure you hit the right one.

On the right of the machine is an LED time display; by repeated pushing of the cursor button you can set the display to show LTC from the tape or from an external source, and the offset can be displayed absolute or relative. Absolute offset refers to the difference between the tape and internal timecodes, while relative refers to the difference between the two taking into account the offset you have chosen. This is a useful indication of how far apart the master and slave are when they are chasing.

The Buttons

As you would expect from a multitrack with inbuilt sync facilities, there are a lot of buttons on the RD8, many dual‑function. Like ADAT, the Stop button has the additional function of disengaging the tape on pressing it a second time, which improves the fast wind time by disengaging the tape from the heads.

The LCD display has a system of menus that may take some getting used to; there is a main menu which allows access to most of the important functions, starting with the sample rate. Here is a feature which many ADAT users will consider a great advance: the sample rate can be set without using the varispeed control. On the Alesis ADAT the sampling rate is set at 48kHz, and if you wish to lower this to 44.1kHz you have to set the varispeed to ‑147 cents, whereupon the display will confirm that you have indeed set the correct value. Fostex have also sensibly allowed pullup and pulldown of the two sampling rates to compensate for the problems that may otherwise occur in film or video transfer situations.

The next page of the main menu allows a MIDI System Exclusive dump of machine parameters, and also writing or reading of a table of contents. The table of contents, which is written in the data area at the head of the tape, is readable by the Alesis BRC, with the exception that the RD8 offers a maximum 25 seconds pre‑roll time, which is rather more than the BRC can manage.

Timecode level and output frame rate are selectable on the main menu; timecode can be enabled for high speed output in fast wind, and user bits can be set. Zone setting allows the machine to function over a limited section of the tape, so you can't accidentally erase other work. The crossfade time for drop‑ins is adjustable in four stages, from 11ms to 43ms (at 48kHz), and like the Alesis ADAT, drop‑ins and out are marvellously tight and glitch‑free.

Further down the menu come MIDI Machine Control On/Off, clock and date set, and tape size. The duration of an ADAT tape is a bone of contention for many people — just how long does a 120‑minute SVHS video tape last in an ADAT recorder? The answer is either just over 30 minutes or just over 40, depending on whether the tape was originally intended for the NTSC or PAL/SECAM market. NTSC tapes are longer because of the higher frame rate, and therefore last longer in an ADAT. If 40 minutes isn't long enough, then 160‑minute NTSC and 240‑minute PAL/SECAM tapes are available that last just over an hour in the ADAT's time frame. You need to tell the machine which you are using, though, otherwise the incorrect tape tension can lead to unreliability. Further down the menu is the LCD contrast, error rate and software version. In general, error rates are amazingly low, and I am sure that it will easily be possible to follow the rate of deterioration of the heads or tapes and plan for replacement before they cause a significant problem.

Many other displays are available when Data Edit is used in conjunction with the RD8's other buttons. After a while, most of this will seem straightforward, although I was initially foxed when pages suddenly became unavailable because a parameter I had set previously made them irrelevant.

To set a locate point (there are 100 available) you have to select the number of the locate memory you want to allocate, then use the XFER (transfer) soft key to capture the tape time. You can edit this value with the cursor and up/down buttons if you wish. I feel that this is an unnecessarily attention‑demanding feature when on other recorders you can just press a button to set a locate point. Much work can be done with just one or two locate points that you continuously reset as you go through a project. Maybe a future remote control will make this easier, and provide instant access to at least 10 of the 100 memories.


The Fostex RD8 is identical to the Alesis ADAT in terms of sound quality and I would be quite happy to use it for any purpose where 16 bits are considered adequate. As to synchronising in a multiple ADAT system, I tried a combination of two RD8s and an Alesis ADAT. Having one Fostex and the Alesis as slaves, the Fostex was always first to catch up and lock, while both Fostexes slaved quite happily to the Alesis. I tried syncing an Alesis with BRC controller to timecode and compared it with the Fostex. From a stopped position parked in the right place, both machines took about five seconds to lock. The only problem I encountered as far as performance was concerned was that the RD8 sometimes stuttered slightly when it started to play. The audio came in, dropped out for a fraction of a second, and then came back again.


Because much of the hardware is common between the RD8 and ADAT, there would appear to be nothing to choose between them on build quality or potential reliability; the difference is in the professional sync options. If you need more than one machine, then the combination of a BRC and several Alesis ADATs may appeal, though you still don't get Sony 9‑pin compatibility with the Alesis system unless you also add an AI‑2. Alternatively, you can mix and match, with a master Fostex controlling a number of Alesis slaves; this provides all the sync options required for routine post‑production and obviates the need for a BRC — unless some of the BRC's unique functions are vital to your way of working. Besides being an impressive machine, the RD8 benefits from the fact that two manufacturers now support the same tape format, lending further credibility to the ADAT system.

Thanks to FX Rentals for supplying the Alesis ADAT and BRC for comparison.



  • Recording format ADAT
  • Tape SVHS
  • Heads 2 read, 2 write, read before write
  • Recording time 40 minutes (180 minute PAL SVHS tape)
  • 60 minutes (240 minute PAL SVHS tape)
  • Fast wind 20 x play speed unwrapped
  • 10 x play speed wrapped


  • Channels 8
  • Conversion (record) 16‑bit linear
  • Delta‑Sigma 64x oversampling; Single convertor per channel
  • Conversion (playback) 18‑bit linear; single convertor per channel
  • Sample rate 44.1/48kHz
  • Varispeed ±6%
  • Frequency response 20Hz‑20kHz ±0.5dB
  • Dynamic range 92dB (A weighted)
  • Distortion 0.009%
  • Crosstalk ‑90dB @ 1kHz
  • Wow and flutter Unmeasurable


  • Connectors Balanced Two DB25 Unbalanced 16 phonos
  • Input impedance Balanced 10kΩ Unbalanced 10kΩ
  • Output impedance Balanced 600Ω Unbalanced 10kΩ
  • Nominal input level Balanced +4dBu Unbalanced ‑10dBV
  • Maximum input level Balanced +19dBuUnbalanced +5dBV


  • Connectors Two EIAJ optical connectors


  • Connectors Two quarter‑inch jacks (locate/play, punch in/out)
  • Remote control unit Wired, model 8312
  • Sync connectors Two DB9
  • Sync capability Master/slave syncing for up to 16 machines
  • Remote connector DB9 for RS422, Sony P2 protocol


  • Timecode input:
  • Level 0.15V to 10V peak to peak, 10kΩ
  • Connector Balanced XLR
  • Reader speed 1/30 to 50x play speed
  • Frame rate Automatic recognition at play speed for PAL, SECAM, NTSC. User must select 29.97 or 30fps for NTSC
  • Formats 24, 25, 29.97 DF/NDF, 30 DF/NDF
  • Timecode output:
  • Level 0V to 3V peak to peak
  • Connector Balanced XLR
  • Formats 24, 25, 29.97 DF/NDF, 30 DF/NDF


  • Connector BNC with 75Ω termination switch
  • Level 0.3V to 10V peak to peak unbalanced, 10kΩ
  • Format Composite (NTSC, PAL, SECAM)


  • Connector Two BNCs (in, out) TTL level
  • Phase differential ±20 ns or less
  • Output jitter ±2 ns or less


  • Connectors IN, OUT
  • Data communications MIDI machine control, MTC


  • Extensive sync and interfacing capabilities.
  • Alesis ADAT compatible.
  • May be used as master with standard ADAT slaves.


  • Same limited recording time as ADAT.


The RD8 is especially suited to the post‑pro market, as all the essential interface and sync systems (including Sony 9‑pin compatibility) are built in.