High speed dubbing (Cassette)

[stacman]

Seem to recall Marantz making one that was dual speed,
Regards, Alan

[TIMTAPE]

Originally Posted by TonyDuell

I had wondered if such machines ever existed.

Do you happen to know of any makes/models?

Don't recall the model but Marantz was one. Ed: sorry already covered.

The Tascam Portastudio did run at 3 3/4ips

[ben]

We should be wary of certain comparisons. The average domestic midi system or twin-tape radio recorder with x2 speed dubbing has little if anything in common with the pro and semi pro cassette duplicators which run at 8x or 16x. These have specially designed circuitry to handle the high frequencies and corresponding heads. They were built to stand up to the kind of use a domestic item would never see in a hundred lifetimes. All the ones I have owned or used coped both sides of a tape in one pass. OTOH, you can find speech-grade units with no top end, and others which give a decent result with music.

Copies made at x2 on many of the domestic units tend to suffer from increased background noise and EQ distortion. At x2, highs will effectively double in frequency and they will be out of range of the recording preamp spec. Some decks seem to cut them off, others do very little if any proper re-EQ-ing - I always found that a good check was to see if there was added sibilance on certain sounds.

Commercial pre-recorded tapes are duplicated in a different way entirely, first by special open reel duplication masters, latterly via digital source loop bins - and recorded onto 'pancakes ' of cassette tape used in loaders. That way the drag from the shell is non-existent, for one thing. HX pro helped overcome the bias problem. The huge bandwidth on this kit is pretty much as good as it gets. The likes of Otari and Lyrec were perhaps the only really known providers of the kit. A good chrome tape made like this is way better than the average domestic recording.

[ben]

Wow and flutter is another factor which sets apart domestic and pro kit. The average dual well deck has pretty low specs to begin with. You then have this W&F to factor in on both the playback and the recording mechs. Add the higher speed, the possible negative influence (drag, flutter, poor azimuth) of the cassette shells, ALC, and any misalignment of the heads, and you will end up with a copy that is a pale shadow of the original.

On the subject of W&F I have somewhere a Sharp boombox with one long capstan which passes through two cassettes loaded in a row. Unfortunately any sonic advantage is lost due to the cheap circuitry and overall budget build!

The only way to get a good tape-to-tape copy at home is to do it in real-time. Use two high quality decks, which have new belts, have been lubricated, are biased properly, are aligned (ideally the azimuth of playback/source unit has been adjusted to the source tape). If the recording deck is 3 head this makes life easier as you can set up the bias and rec level and check the recording in virtually real time, comparing with the source. It is possible to get really good results this way.

[TIMTAPE]

Originally Posted by ben

We should be wary of certain comparisons. The average domestic midi system or twin-tape radio recorder with x2 speed dubbing has little if anything in common with the pro and semi pro cassette duplicators which run at 8x or 16x. These have specially designed circuitry to handle the high frequencies and corresponding heads...

Even at only x2 dubbing speed, for good copies the consumer dual cassette machine needed bias frequency, repro curve and record pre emphasis curve increased by an octave. If that was not designed in, yes the result would have been less than optimal. I'm pretty sure there were consumer machines with these features, with the changed EQ settings automatically switched in when High Speed dubbing was selected. Of course there would be no need to design in two different bias frequencies. The higher one would have also sufficed for normal speed recordings as it did with multi speed open reel tape machines and the linear audio on VCR's.

I still believe that high speed dubbing did not of itself lead to poor results, just poor implementation of it, as well as the difficulty in monitoring the audible result until it was possible to do so after the dub was made. The results of professional VERY high speed duplicating seems to bear this out.

[ricard]

It seems to me that the main limitation of high speed dubbing would be a) the amplifier and b) the recording bias frequency. I would think that the primary limitation of heads would be gap length, although certainly there are primarily magnetic losses at higher frequencies, but the gap length itself would not be a limitation as the high end frequency response would be limited by the relationship of the gap length and tape speed to the frequency, which remains constant.

With consumer machines rolling off at about 14 kHz or so, a low level amplifier designed to operate up to 28 kHz is not particular hard, it just needs to be properly designed. Similar for bias frequencies, I would think around 50 kHz would be de facto for consumer decks, and 100 kHz is not an especially high frequency.

Drag etc would be worse, but on the other hand, the effect of a flywheel would tend to be increased when it is spinning faster, so there is potential for the actual speed to be more stable at the higher speed in terms of wow and flutter.

[TIMTAPE]

I agree Ricard.

Modern silicon small signal transistors can go way above audibility into the RF region. The practical limitation is set by the LPF caps which are revised down in value.

Re heads, gap length relates not to frequency as such but wavelength on the tape. With higher frequencies, as I understand it, head inductance becomes the issue. Inductance needs to be low enough to not attenuate the higher frequencies. All the play and record heads from the x8 or x16 high speed duplicators I worked with had significantly lower inductance than standard cassette heads.
For consumer decks with a x2 option I guess the heads would have needed to be a little different in spec from standard cassette heads but not nearly so different as for much higher copying speeds.

Somewhere I have some Otari 1/4" open reel duplicator playback heads which also have a much lower inductance than standard open reel playback heads. They were designed to copy standard 3.75ips tape masters at x8, meaning a play back speed of 30ips. For x8, 7.5ips tapes played at 60ips.

Bias frequency needs to be 4 to 5 times the highest expected frequency captured to avoid beats. The Otari cassette duplicator slaves I used to service ran at x8 and had a bias oscillator frequency of about 700kHz.