At last — a truly affordable way of digitally mastering your recordings.

Before launching into the details of this particular unit, it's worth looking at the concept of digital recording in general for the benefit of those readers who are not yet aware of its potential. Sony really started the ball rolling with their F1 PCM processor as far as the affordable market was concerned and, in principle, the system converts an analogue audio input into a pulse coded digital format which may then be recorded onto a standard domestic video recorder. When the recording is replayed, it again passes through the processor and a high quality stereo audio output is derived from the digital signals stored on the video cassette. It sounds simple, and indeed it is from the operators point of view, but the technology behind this processor is incredibly sophisticated.

The 501 is a downmarket version of the F1 made possible by custom IC design but what corners if any have been cut in producing this new unit remains to be seen.

Format

The incoming audio is digitised into either a 14- or a 16-bit code depending on the user's requirements. Though the 16-bit version gives better sound quality in terms of noise, dynamic range and distortion, there's an industry standard that operates on the 14-bit system, hence the option. Also the error correction system is more effective on the 14-bit format, but more of that later.

Converting an analogue signal to digital means sampling, and the 501 uses a sampling frequency of 44.1 kHz, giving a frequency response of 5-20,000Hz. This coupled with the 16-bit sampling resolution means that you have, in theory at any rate, got the capability to equal the quality of compact disc recordings but with an even better frequency response. As with compact disc, the noise and distortion is very low and speed effects such as wow and flutter are immeasurably small. As the sampling is linear (the input is sampled into steps having equal increments), some pre- and de-emphasis of high frequencies is used to further improve the noise performance. This involves boosting the level of the high frequencies on record and cutting them by the corresponding amount on playback which leaves the sound back where it started but reduces the high frequency noise content due to quantisation. Quantisation noise is caused by differences in the input and output waveform due to the digitised waveform consisting of a finite number of steps. Each point on the analogue waveform is digitised to the nearest step and though these errors are small on a 16-bit system, they are nevertheless present and they show up at the output as noise. Some pre-recorded material may not have been recorded using pre-emphasis and in this case, the 501 is equipped to detect the fact and automatically switch out the de-emphasis on playback. A status display lets you know whether the machine has switched this facility in or out.

Errors

There's another type of error which can be much more serious and this occurs when data is lost or corrupted due to faults on the tape or wear in the VTR. In an analogue machine, such problems manifest themselves as dropout which usually means a momentary drop in level with perhaps a corresponding loss of high frequencies. In a digital system however, what goes onto tape is a stream of digital code which is really a series of numbers describing the instantaneous height of each sample. (There are other things embedded in the code which are to do with the housekeeping functions of the 501 but we don't need to know much about that at this level.) In normal circumstances, these numbers will not deteriorate or become corrupted with tape wear, as a digital one should still be recognisable as a one and a zero as a zero which is why the quality of digital recordings can be so high. If however there's a catastrophic failure of even a small section of tape such as may be caused by a fine scratch or a piece of dust, things get more serious. What happens then is that numbers do get corrupted and if these are decoded at the other end of the system, they can manifest themselves as very nasty clicks. Accepting that tape is always susceptible to localised surface damage, the manufacturers have built in a couple of neat tricks to get round this problem.

The first such trick is that of interleaving data words when they are recorded onto tape. If there's a patch of surface damage on a tape, you might reasonably expect that the data corruption would occur over several successive data words resulting in a totally duff section of information, but interleaving helps to get round this. As I understand it, the data words are not just recorded onto tape one after the other but are stored for a short time in a block of memory within the 501 where they are in effect shuffled according to a pre-arranged plan and then recorded onto tape. At the replay end of the system, these words are unshuffled again and so it's more likely that a corrupted word will be surrounded by uncorrupted ones and in this case, the error correction software can make an educated guess at what the correct word should be by looking at the ones on either side and then interpolating.

In the event that the corruption is so severe that the error correction circuit can't handle it, the output will again contain clicks but this is normally experienced only when using either damaged or cheap tapes. But the 501 has yet another line of defence in that it can be set to automatically mute these corrupted sections and, if they're very short, you may well not even notice. If things are really bad however, the output quality will suffer and no amount of electronic trickery will restore it to its former splendour.

"...as I left my TV and aerial connected, I found that I'd somehow ended up with the soundtrack for an episode of 'Dynasty' recorded on the analogue audio track."

The Unit

This is not a rack mounting piece of gear as you might first imagine and it's not 19 inches wide. It looks more like a piece of hi-fi equipment than a studio processor and this is fair enough as there must be a huge potential market in that area now that the cost has come down. Measuring a mere 430 x 80 x 350mm, the PCM-501ES is finished in black and sports relatively few controls. On the rear panel are phono connectors for the Audio Input and Output, phonos for the Video In and Out and two additional phonos for Monitor Out and Copy Out.

Amongst the front panel controls, there's a mains power switch (which we'll take as read). Just below the mains switch is a stereo headphone socket which may be used for general listening or simply to check your recording, and there is of course a volume control for this facility. Next comes the OVC section which requires a bit of explanation. This stands for Optimum Video Condition and is a method of matching the PCM processor to the recorder for optimum recording accuracy using the Tracking button and the Picture Control knob on the VTR itself. At this point I got rather worried because my VHS VCR didn't have a Picture Control knob! A triple LED display is provided so that you know when you've got the best match.

Firstly, the tracking itself is optimised and for this, the lower LED signal strength meter doubles as a tracking indicator; the further to the right the display goes, the better the tracking and this is accomplished by adjusting the Tracking control on the VCR with a pre-recorded tape playing. Once the tracking is right, the OVC control and the VCR Picture control are adjusted whilst observing the OVC display until the best match is achieved. As far as I can tell, this procedure looks for a condition of minimum corrupted data.

"...this is a remarkable piece of equipment for the price..."

As mentioned earlier, there's a requirement for a 14-bit as well as a 16-bit format, and so a front panel pushbutton switch gives access to either option and the current status is displayed in the level meter window. In playback, the machine recognises which format it is playing and switches accordingly.

When copying from one tape to another using a pair of VCRs, the Copy button and Copy output allow the data to be copied from one machine to another with no loss in quality. The indicator section shows when the copy function is invoked. However, if you have a prerecorded tape which incorporates a Copy Prohibiting code, the machine will recognise this and will not allow you to make copies... that's all we need, a machine with morals!

Auto Playback Mute has been briefly mentioned before and when it's enabled, it will mute corrupted sections of tape rather than let the resulting clicks and noise through to the output. On really bad tapes you may choose to disengage this facility as it will probably annoy you by muting out significant chunks of the programme. The muting indicator lights whenever an event is detected which would cause the muting to operate whether it is switched in or out.

When you're recording it's often useful to be able to record a section of silence without stopping the tape, and this may be done using the Record Mute button which operates only for as long as it is held down.

The metering section is very important on a digital recorder as there is no headroom above 0VU; you simply run out of bits and the distortion is both sudden and nasty. For this reason it's best to set the levels such that peaks read a few dBs below 0VU and then you'll still get a good signal to noise ratio without having to worry about headroom. In the event that you do go over the top, the Over indicators will light up to remind you of your sins.

The meters themselves consist of horizontal fluorescent bargraph meters with a peak reading segment held on a long time constant so that short transients are registered long enough for the operator to inspect and worry about.

"On making a recording from one of my better compact discs, I couldn't tell the result from the original and even careful listening through a good pair of headphones revealed no apparent flaws, noise or distortion."

In Use

Connecting the system up to a suitable VTR presents no problem as you only need a couple of phono to phono leads and another two pairs of phono leads to plumb into your audio system. After all this talk about error correction and muting and data corruption, I thought that I'd come across all kinds of problems, especially as I was using a VHS recorder rather than a Betamax but I needn't have worried.

The manual is a little vague about the setting up of the OVC control and I'm afraid that I can't help you much as it refused to read anything other than optimum, as did the tracking from the moment I set things running. In fairness, the tracking control is only likely to need adjusting when you try to replay a PCM recording made on another VCR.

Operation is unbelievably simple and the results showed no sign of muting and no error lamps flashed at all. On making a recording from one of my better compact discs, I couldn't tell the result from the original and even careful listening through a good pair of headphones revealed no apparent flaws, noise or distortion. However, the PCM system only uses the part of the tape normally used to carry video information; the audio track is quite redundant, and as I left my TV and aerial connected, I found that I'd somehow ended up with the soundtrack for an episode of 'Dynasty' recorded on the analogue audio track. This could be replayed via the TV set at the same time as the PCM encoded stereo track was being played through the hi-fi system.

Not only that but when the PCM recording is played back, there's a black and white representation of the changing bit patterns shown on the TV screen which is something like the pattern you get when loading a Sinclair Spectrum. Neither of these novel features affects or detracts from the operation of the PCM system in any way whatsoever but I just thought I'd mention it.

I was hoping to tell you what audible effects the automatic error correction and muting had on the sound quality of the programme material but at no time did anything occur to make me think that it was even working. Logically I suppose that some minor tape flaws are inevitable and so the error correction must be called into action for short periods of time on a fairly regular basis but it's quite transparent.

In conclusion on this point, I would think that any reasonable well maintained VCR fed with reputable tapes could be expected to work perfectly and it probably takes a badly abused tape to show up any problems at all.

"I don't know what compromises have been made in sound quality (if any) to keep the price as low as this but I certainly can't find anything to complain about here."

Conclusions

I don't know what compromises have been made in sound quality (if any) to keep the price as low as this but I certainly can't find anything to complain about here. True, it's impossible to edit on this type of system and you don't know to the nearest second when a recorded piece starts, but for pure and simple mastering, this doesn't matter unless you subscribe to the 'mix with a razor blade in one hand' school of thought. The indeterminate start time is caused by the tape loading system which has to pull a loop of tape from the cassette and wrap it round the head and you don't know from which reel the most tape will be unwound. This combined with the very low tape speed of video cassettes can lead to errors of several seconds so you must leave a safe gap between songs to save chopping the end off the previous track when you add on a new one. This precludes this PCM system for spinning in bits of a song during a mix, which is a shame, but the limitation is caused by the VCR, not by the PCM processor.

Because the sound quality does not change or deteriorate with repeated playings (unless you really thrash the tape to destruction), this medium is ideal for providing the master for a real time cassette copying facility and as many VCRs have a remote transport control facility, it's possible to rig up a partially automated system.

Using the Copy facility, copies of the video cassette can be made using the PCM processor and two VCRs without incurring any loss in quality whatsoever, which is quite impossible when you copy ordinary analogue tapes, no matter how good the machines are.

To sum up then; this is a remarkable piece of equipment for the price and you can also use it as part of your hi-fi system in its off duty hours. The sound quality is at least an order of magnitude better than the next weakest link in the chain so even if there are some compromises, you're not going to notice them if you are working from an analogue multitrack. The only problem is that using the PCM-501ES soon makes you aware of the noise generated elsewhere in your system.

The PCM-501ES costs £446.00 including VAT, and for further details, see Turnkey, (Contact Details).