[BC] Cassette deck maintenance

[Jeff Glass]

In a message dated 2/17/2010 12:18:07 PM Central Standard Time, broadcast-request at radiolists.net writes:

>No, you are NOT comparing the record to the playback-you are comparing the
>OSCILLATOR to the playback! Let's say the recorder is running slow-it would
>record the tone slow and pay it back slow-COMPARED TO THE OSCILLATOR! Same
>thing if it was running fast. You are using the OSCILLATOR as your
>standard-and as long as that standard isn't changed during and after the
>recording, the standard can be ANY frequency!

>-D

This makes no sense. I agree with Steve and Dave Dybas. If you record a tone of 1 kHz at 7.5 ips, it will play back on-frequency only if it's played back at 7.5 ips. If you have a machine that you want to run 1-7/8 ips and it's really running at 1.5 ips, what do you think a tone is going to sound like if recorded at 1.5 ips and played back at 1-7/8? It's going to be too high in frequency; it will only play back on-frequency if played back at the same speed as used for recording; in this case, 1.5 ips; just like if you recorded a tone at 7.5 ips and played it back at 15 ips; it's going to sound high in frequency. This concept becomes crystal clear when you look at an extreme case of tape speed error.

If you have a NAB reference standard tape (recorded at standard speed) with a 1k tone, you can compare that 1k tone with an external oscillator and adjust the speed to make the playback match the oscillator. Doing so gives you a machine that runs at reference standard speed.

Comparing a recording of your oscillator (made on the machine under test) to the oscillator itself, will only verify that the machine is playing back at the same speed as when you recorded your test oscillator output.  This does not tell you that the machine is running at standard speed.

This seems pretty clear to me. What am I missing?

Jeff Glass
Northern Illinois University