High speed dubbing (Cassette)

[SiriusHardware]

Probably a silly question, but: During a recent offline conversation the subject of twin cassette decks as hi-fi separates and in high end cassette portables (Ghetto Blasters) came up.

A little while later it occurred to me to wonder how the high speed dubbing feature on those actually worked. The first bit is obvious, both decks have to run at the same higher speed.

But let's say the source tape has audio on it going up to 15Khz - if played and recorded at three times the speed, then surely every part of the path from the replay head through to the record head has to be overdesigned and capable of handling 45Khz just in order to be able to handle that one little-used feature. And if the bias was AC, which it would be on any decent machine, didn't that need to be raised in frequency as well?

So what really happened on these machines? Was the loss of the top end just accepted as a penalty for using high speed dubbing?

[paulsherwin]

Yes. There was never any pretence that these machines produced hifi copies.

[cmjones01]

I once saw a review of some midi system or similar with this feature. It explained that dubbing was possible at normal or high speed, "depending on whether you intend listening to the copy or not"!

Chris

[dave walsh]

I like that review Chris lol. I've had a number of twin cassette decks over the years including the high end one [mentioned in another thread] that could record over four cassette sides instantly and simultaneously with no break!. That and many cheaper machines, including a Sony RCD-W100 that I'm using currently, offered H S dubbing, the subject under discussion here but I've never used that feature at H S figuring, without any technical checks that there would be an inevitable loss in quality.

I always wondered why manufacturers decided that would be a handy feature to offer in the first place, unless it was a response to the mixtape culture of the 90's? What had always been a tape enthusiast practise, going back to r to reel, then became a sort of social norm amongst the tape swapping/introduction Boombox Culture. Everything [particularly with the "yoof"] now tends to be a matter of instant gratification and perhaps that was an addition for the impatient or those involved in counterfeiting commercial tapes? [I draw a significant distinction between that and Bootlegging!]

Dave W

[emeritus]

I have two 1980's music centres, a Philips and a Sony, that have high speed dubbing. Can't say I have needed to use them myself, but in the pre- streaming era could hve been useful for things like making copies of lectures for those unable to attend in person. This was done in the 1980's and 90's for allowing students not based in London to benefit from lectures delivered in London.

[TIMTAPE]

Originally Posted by SiriusHardware

But let's say the source tape has audio on it going up to 15Khz - if played and recorded at three times the speed, then surely every part of the path from the replay head through to the record head has to be overdesigned and capable of handling 45Khz just in order to be able to handle that one little-used feature. And if the bias was AC, which it would be on any decent machine, didn't that need to be raised in frequency as well?

Yes everything had to be raised in frequency as you say. There was some compensation in that while they required heads and electronics which could handle higher frequencies, the previous lowest frequencies werent required to be duplicated. As I recall these consumer machines usually only copied at twice normal speed which wasnt such a stretch, and if set up correctly did a good job, at least to my ears.

Cassette tape duplicating was done commercially at very high speeds, including over x100, using specially modified open reel machines designed for large cassette width tape pancakes, and smaller organisations used "in cassette" high speed duplicators, usually at x16 or x8 speeds. The results could be excellent but depended on how well the machines were maintained, and set up for the blank tape stock used. Duplicators could suffer a hard life because of the kms of tape run through them in a very short time, and head life could be short. Partly for these reasons, and because the smaller machines were often operated by unskilled, the results were often poor, so high speed duplicated tapes earned a bad reputation which was not really deserved in my opinion.

[electronicskip]

I am pretty sure that the Amstrad SM104 VLT Hifi system I had back in the day had Hi speed dubbing on.

[Richard_FM]

The Yamaha tape deck I have has high speed dubbing setting but I've only used it once or twice.

I always assumed quality would be compromised, but it was interesting to hear serious artists sounding like the chipmunks!

[SiriusHardware]

Thanks all, and thanks for confirming what I suspected. In fact this came up in a discussion about various cultural trends in the early eighties, one of which was the playground phenomenon of computer software 'swapping'.

No doubt the twin cassette deck was the playground pirate's tool of choice for this job, but when high speed dubbing was used, as it probably was where available, I can't imagine the copies getting through many generations before becoming unusable.

[TIMTAPE]

Originally Posted by SiriusHardware

...No doubt the twin cassette deck was the playground pirate's tool of choice for this job, but when high speed dubbing was used, as it probably was where available, I can't imagine the copies getting through many generations before becoming unusable.

Yes and generational loss is the key. With analog, there is always generational loss regardless of the dubbing speed. Even the first analog generation has an equal degree of loss as any subsequent generations, and the losses are of course all additive.

There were further problems with high speed dubbing in that we couldnt properly monitor in real time by ear the sound of either the original playback, or the dub, as the dubbing progressed, as there were now humanly inaudible frequencies at play.

Years ago much of my work was service of high speed cassette tape duplicators. The only way I could monitor in real time the frequency response of the dub in progress was with a specially modified cassette, with a special high speed playback head mounted inside it. The output was displayed on a CRO. It was especially useful for adjusting record bias. I found that loss of highs in duplicators was so often due to an overbiased high speed record head.

Photo attached. Note: the special 3rd head is partly obscured under the toggle switch which allowed switching between side 1 or side 2 playback of the tape copy.

Azimuth alignment of both the master playback head and the record head was also tricky because again you couldn't align azimuth by ear, only by instruments.

Really there were multiple reasons why high speed dubs could sound indifferent but they weren't inherent to the high dub speed as such.

[paulsherwin]

The quality produced by commercial high speed duplicators improved noticeably with the introduction of HX Pro variable bias technology in the 90s.

[kalee20]

You could at halve the dubbing time without speeding anything up, if you had a full-width erase head, and record and replay heads which spanned the width (total four channels). One 'side' would be replayed backwards from the master, but it would also be recorded backwards onto the copy, so it would play fine!

[paulsherwin]

That is what commercial duplicators do.

[ms660]

I used to service the Sony CCP-13 duplicators many moons ago, frequency spec was only possible with good quality cassettes if you were lucky.

Lawrence.

[TIMTAPE]

I found those Sony CCP duplicators generally well made and with good performance. The ferrite heads seemed to last well. One small but frustrating detail was the entry tape guide on the left side of the erase head was not rugged enough and the edge of tapes would quickly cut a downwards groove in it, leading to misaligned tape travel on both master and slave decks. What might have worked for a normal cassette deck was inadequate on a machine used at 16 times normal speed.

[julie_m]

All the "high speed dubbing" feature used to do on those systems, was bypass the speed regulator; thus letting the cassette motor run full chat. This pushed up the frequencies involved, possibly beyond the ken of the head gaps.

Did anybody ever make an "all-in-one" stereo system capable of running the cassette at 15/11 times normal speed, thus allowing for taping an LP by playing it at 45 rpm?

[TonyDuell]

Many of the 'ghetto blaster' units had one motor that drove both cassette decks via a rather long belt. Therefore, apart from mechanical tolerances in the diamters of the capstans and flywheel grooves, the 2 decks would always run at the same speed as each other.

I have a Panasonic RX-FT510 that is built like that. The high-speed dubbing feature changes the speed control resistor connected to the motor (it doesn't totally short out the speed control), switches a resistor in parallel with an existing one in the equalisation network for each channel and seems to slightly reduce the bias frequency (which seems totally wrong to me!). The service manual is available on-line somewhere (as I downloaded it when I got the machine) but I am not sure where.

[G6Tanuki]

Yes they generally used one motor so speed-variation when in high-speed dubbing mode wouldn't hve been a problem.

And as mentioned the general use for the dubbing was to copy commercially-bought cassettes (both audio and computer-game: the dubbing worked OK for Sinclair ZX81 tapes!) for your schoolmates or to copy off-air recordings (we all used to sit around early on sunday evenings taping the tracks we liked off the Top 40 before turning the radio off so we didn't hear 'Sing Something Simple').

I've got here a must-be-from-the-early-90s Aiwa "Stereo radio cassette carrying component system CA-W72" [a typical cheap ghetto-blaster] that has the second [play-only] cassette deck marked "High Speed Synchro-Dubbing". Back in the 1990s I used to use it to record the sunday-morning GB2RS News on 80M while I was out at work - I had it hooked up to my RA217 and on a timeswitch set to start recording about 10 minutes before the scheduled timeslot - then I'd dub that week's news as a followon to last week's. The news didn't always start on-time and was of variable length so this was a convenient way to archive it. The 'compilation tapes' - like all my cassettes - are long gone.

[paulsherwin]

There were a few hifi decks marketed which optionally ran at 3.75ips for suppposedly higher quality. They were never very popular, as conventional cassette performance was getting pretty good by that stage and the halving of running times was a serious drawback.

[TonyDuell]

Originally Posted by paulsherwin

There were a few hifi decks marketed which optionally ran at 3.75ips for suppposedly higher quality.

I had wondered if such machines ever existed.

Do you happen to know of any makes/models?