I have a DCC170 that must be spinning faster than normal because a 90 minute cassette is short by about 2 minutes while in other DCCs the same tape I get about 92 minutes. My question is how does this affect playback from recordings on a DCC that is spinning faster or slower when played on other DCCs?
Within a certain tolerance, DCC players dynamically adjust playback speed to the tape.
That makes sense because I noticed on the DCC731 there is a speed adjustment that seems to only affect analog tapes but not digital tapes.
When you record a tape or when you play an analog cassette, the microcontroller in the DCC recorder sets the tape speed to a nominal value which can be adjusted. The tape speed should be 1 7/8 inches per second to be compatible with analog decks.
When you play a DCC cassette, the tape speed is varied continuously based on the bits that come from tape; the bits are decoded at exactly 384 kilobits per second to an audio stream of 32, 44.1 or 48 kHz, with crystal-accuracy. The playback speed will match the recording speed, regardless of which deck recorded the tape. So if your 170 can only record 88 minutes on a 90 minute tape, the playback recorder will play that tape in 88 minutes too. If you would re-record the same tape on the slower recorder, you would get the full duration without loss of quality.
===Jac
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Very interesting. Thanks.
So can we slow it down to get 110 minutes out of a 90 minute tape? 
Incidentally today I adjusted a DCC730 that was running at 3215 Hz from an analog calibration speed tape that should be playing at 3000 Hz. So I adjusted it down, and now it is perfect.
I was flabbergasted that the speed deviation was so big!
I listened to an ACC tape before I made the adjustment, and indeed, there were smurfs/stroempfen (or chipmunks depending where you live) singing.
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I’m sure every recorder has some tolerance in what kind of tape speeds it can deal with on playback. A few percent of speed derivation is probably okay but running the tape at 82%? I’m going to guess that at that rate, the quality of the tape is going to be very important. It would be an interesting experiment anyway. Go ahead and try it! Make sure you have an analog test cassette to reset the speed back to what it was. And keep in mind, there are usually 2 speed controls, one for the A side and one for the B side.
===Jac
Unfortunately the only DCC I have to experiment with no longer records.
Do you know why it doesn’t record anymore? I read that a lot!
It has a bad head. I was able to test this by swapping the head temporarily.