Home Recording With Digital (Part 2)
Overdubbing With The Sony PCM F1
The Sony PCM F1 is a 16-bit digital processor that was actually designed for the domestic market, yet which has a performance superior to most analogue recorders. So much so, that it is quickly becoming a legend in its own time and finding much use by professional recording engineers, for all manner of applications.
As described last month, the PCM F1 consists of an analogue to digital (A/D) unit which feeds a video tape recorder (VTR). Sony's own Beta format seems best suited. The intended use of the processor is for straight stereo recording but the unit has one unsung feature which will allow overdubbing in an analogue form, provided a second VTR is available. The unit is conveniently divided into two sections electronically speaking. The A/D circuitry is used to record or 'encode' with the separate D/A for playback. The metering on the unit functions permanently on the output of the digital to analogue converter, thus reading and displaying the analogue output signal levels of both channels.
Figure 1 shows the interconnections required for analogue overdubbing. Once encoded the playback VTR is decoded into analogue form and passed to a suitable mixer output. The musician(s) then monitor the output of the mixer either in the control room or on headphones for the new, overdub inputs. The old and new mixed feed is then processed onto the second VTR where it is re-recorded.
I have done three or four journeys around the 'loop' and although the signal changes from analogue to digital and back to analogue form many times, the results are still an improvement over standard analogue 'bouncing' techniques. Of course, there is no 'drop-in' facility as is possible with conventional multitrack recording.
In all overdubbing, a run-in countdown is needed, such as 'one, two, three, four' recorded at the start of the first take or layer. This then cues in the performers during any subsequent overdubs. After the final 'mix' the skilled use of a pause control can remove the count-in when a final digital 'copy out' dub is made (assuming there is no reverberation overhang into the music from the last word of the count). If there is, the alternative is to end your count on 'three', with a silent 'four' to create a space at the start of the take. This tip applies equally well to all recording - analogue as well as digital.
Connecting up the machine and general operation needs some thought. As mentioned earlier, the PCM F1 meters the playback VTR feed only. Also, your mixer will need peak reading meters to monitor the combined output signals, because VU meters are inherently sluggish, and are not designed to read very fast transients, if however, your VU meters are augmented by a peak LED of known triggering level (ie +6dB) these will suffice.
There is a further complication however. The F1 meter will show the pre-emphasis that has been applied to the audio signal prior to D/A processing. There is around 7dB signal boost at 10kHz compared to the level at 1kHz which means that, should you have not allowed for this when setting the nominal record level at which you are working, you will hit the digital 'crunch point' very easily if there is any noticeable HF (high-frequency) content in your music.
My experience shows that it is best to take your 'zero level' on the mixer as -20dB reading on the F1. Working with peaks around 8dB above this (-12dB) is then sensible as the F1 dynamic range will be fully used due to the metering pre-emphasis. You have in excess of 90dB to play with, remember!
In the setting of levels the requirements are as follows; how you implement them will depend upon your particular mixer. You'll need a source of tone, ie. an oscillator giving 1 kHz at a suitable level. Initially, before any recording takes place, couple in the oscillator to the channel to be used for the F1 playback and set everything up so that the mixer's meters read zero VU (or whatever operating level you have chosen) and the PCM F1 reads -20dB. Record the 1kHz tone for several minutes.
Then replace the oscillator connection with the playback VTR - your mixer should still read 0VU when the recording is actually replayed. If not, readjust your gain levels and for confirmation copy the tone onto the record VTR. Play this back and check that the level around the loop is still constant.
This operation highlights the beauty of digital recording: there are no levels to be set between the playback VTR and the F1, nor are there problems with azimuth head adjustment. The test tone you've recorded will be as 'pure' on playback as the original, with no modulation noise or side effects that we've come to accept from analogue.
The accurate setting of record levels 'around the loop' ensures that there is no unnecessary reduction in level around the system, or worse still an increase, as this could quite easily lead to overload and system 'break up'.
When recording is underway and you are playing back your first layer, the new inputs can be controlled and the whole viewed on the mixer meters at the same time as being audibly monitored on speakers or headphones.
It could be that as you add more overdubbed layers the signal level begins to approach the overload point. The solution is to simply drop the level of the mixer output a few decibels, as this won't make too much of an audible difference as far as noise enhancement goes.
Another possible tip is actually to record each transfer one dB or so below the original level to compensate for level increases from the overdubbed layer. In theory, two identical instruments in sync and in-phase would produce a 3dB level increase when combined onto a recording. In practice, this is unlikely to occur so the average increase is slightly less than this.
It would seem to me that with the Sony PCM F1, or the alternative PCM 701, and a couple of Beta format video recorders, the capital outlay is certainly justified, considering the class of performance obtained - certainly compared to an eight track with stereo mixdown on reel to reel. Bear in mind that you may well be able to borrow the family VTR and that of your next door neighbour!
VHS machines can be used but some suffer from a curious 'run back' feature which you would have to allow for in any cueing. Mixed Beta and VHS machines will obviously mean alternate record and playback use, needing the connections to be re-jigged each time. To overcome this you can quite easily make up a switching system, as I have incorporated in the portable overdubbing suite I use for mastering Whitetower records.
Feature by Mike Skeet
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