Spectrogram and 60 Hz hum

[Dan McDonald]

I've always used the spectrum analyzer to find the 60Hz point to check whether the speed is right on tapes, but recently tried the spectrogram and it seems very useful. If you zoom in and adjust the contrast, you get a very specific line at the 60Hz point. I've shown the range between 50 and 70, and zoom in to just a few seconds. With the contrast adjusted, you get a distinct line (in my case a black line standing out very clearly from the red, yellow, etc. colors that are visible.

Is that a reasonable thing to do for this kind of work or is there a reason that the spectrum analyzer is better? It certain seemed to work well for me.

Dan

[Craig Maier]

Hi Dan,

It is a good way to do it because you also can view changes in the frequency vs time that way. You can further increase the resolution by upping the fft size (especially with the Forrensics version) and adjusting the "Inrease Resolution" slider control towards the left (frequency). That way, you can really zoom-in on the hum signal.

Craig

[Dan McDonald]

Thanks Craig. I was hoping you'd say that - it seems much more specific and easier to find the 60Hz signal. When I was doing it, I was getting a thin black line right at 60 Hz and you could just click right in the middle of the line and get the signal frequency, which made it very easy.

[Craig Maier]

Hi Dan,

And, if the recording was made with a battery operated recorder, you can view the signal over the entire file and observe if the hum signal drifted as the batteries wore down over time. Thus, you can compensate for that with the non-linear speed change filter if that problem exists.

Craig

[Dan McDonald]

Yes, that's very helpful. Cassettes tended to be pretty steady up til the end of the battery life, though, right?

[Craig Maier]

Dan,

Yes, they did use a closed loop control system for the capstan motors. But, some of them were very simple flyball mechanical type governers (located inside the motor housing) and they would drift a small amount (call it a second order effect) as the input Voltage fell. Put another way, they were not perfect regulators - - - todays analog capstan driven tape regulators are 100 times more precise compared to the early Compact Cassette versions.

Craig

[Dan McDonald]

Ok - good to know. I remember my first cassette recorder. My dad bought it for me. He had gotten me a portable open-reel and the batteries lasted about 20 minutes or so - a very nice recorder when the batteries were working, but started going weaker very fast and then the tapes just weren't very good. The cassette changed all that. I was completely shocked when I could record for half an hour and it all sounded the same!

[Craig Maier]

Hi Dan,

Yes, I remember those early battery operated tape machine days - - - both reel to reel and then Cassette. Around 1969, I had purchased a monophonic Sharp tape deck (RD-403 as I recall) that ran on a pile of D cells (4 cells, I think). It was mono, but I used to sneak it into rock concerts along with a good mic and it would record for several hours before the batteries started to die. I would sneak in the tape deck and my girlfriend would sneak in the mic and cable. It was great, all things considered!

Craig

[Dan McDonald]

Still have those tapes?

[Craig Maier]

Dan,

Sore subject - - - I had all of my cassette tapes in my car, including the concert tapes (groups like Mott the Hoople, 10 Years After, Beach Boys, Association, and so forth). And, you got it - - - somebody broke in and stole all my tapes. My college was in Newark NJ (Newark College of Engineering / New Jersey Institute of Technology - - - a real bad neighborhood. Lucky that my body was not stolen and sold for its mineral content - - - but I digress.

My stupidity on that issue pisses me off to this day. And no, I had not made a transfer to reel to reel at that time. I just kept listening to the originals. Unfortumate, because I had the equipment to transfer it to a Magnecord pro deck, but did not think ahead so I really did two stupid things.

Based on that, I would not hire me!



Craig

ps - the odd thing is that the person who stole the tapes would not have known that they were one-of-a-kind. It is possible that they just used them as blanks and taped over the music.

[Dan McDonald]

I had a tape I made of Larry Coryell - the jazz guitarist - loaned it to a guy to 'listen to' and it never returned. Same story - no backup.

When something that only exists in one copy disappears for no good reason, you do tend to have a sore spot.

Dan

[Craig Maier]

I had a tape I made of Larry Coryell - the jazz guitarist - loaned it to a guy to 'listen to' and it never returned. Same story - no backup.

When something that only exists in one copy disappears for no good reason, you do tend to have a sore spot.

Dan

Yeah, you'd think that after ~ 40 years I would have gotten over it. hmmmm -