I currently have over 150 7" reels of tape made at 3 3/4 ips to record and save as mp3 files. Majority are music tapes.
Wondering if I can save lots of recording time by recording at 7.5 ips and then editing it by changing speed back to 3 3/4 before exporting to mp3 ???
Will I lose anything by doing this ---- if successful I can cut my recording time in half.
In theory, yes you can, but you will lose some sound quality. The most likely effect is that it may sound a bit muffled.
Note also that MP3 is a lossy format (loses some sound quality).
If you decide to go for best quality at x1 speed, then it may be a good idea to make a backup copy of each tape in WAV (or FLAC) format, so that if you decide that you want to edit / process / re-encode or anything else in the future, then you have a high quality copy to work with rather than re-using the lower quality MP3 as your starting point.
Yeah… If you want the best quality don’t do it. And, you’d only be saving “machine time”, not man-hours… You can do something else while keeping an eye on the tape machine…
On the other hand, if the best quality isn’t that important to you or if the tapes aren’t that great to begin with, you can do an experiment. Try it! …Digitize the same song at normal speed and double speed, and compare the results. Or, just compare the double-speed recording to the original tape.
Here are the issues - The “traditional” frequency range of human hearing is 20Hz - 20kHz (some of us older guys can’t hear to 20kHz 
). Good hi-fi equipment can go up to about 20kHz, but the electronics in your tape machine might not go above that range. If you double the tape speed, the highest frequencies will be around 40kHz and they may be lost. Similarly, your soundcard may not pass ultrasonic frequencies, and digitally the signal is mathematically limited to half the sample rate (so you’d need to record at 96kHz).
Another minor issue is the [u]NAB Tape Equalization[/u]. With the frequencies shifted, the playback EQ curve is being applied incorrectly. It’s not a big problem because the overall slope of the curve is correct, but it is not a straight line so it’s imperfect. And of course, the curve isn’t specified above 20kHz so we don’t know what your particular machine will do (if the ultrasonic frequencies get through at all).
Many thanks for both responses — Was just a thought of how to complete these tapes a bit more quickly, but seems best just to continue playing them at the proper speed and not play around with compromising quality. Appreciate your input. Alan [Advert removed by moderator]