Problems with frequency response
Posted Jul 27, 2021, 8:16 AM UTC
613
That is absolutely comb filtering. You've got two signals a couple of milliseconds apart causing phase cancellations at frequency multiples. What's kind of strange is that the multiples are 1000 hz apart, but fundamental at 500 (notice the dips at 500, 1500, 2500, 3500, etc), but that still tracks with how comb filtering operates.
Since the fundamental is at 500 Hz, that means you have the same signal that's off by 2ms (1/500 cycles per second = .002 seconds per cycle). My first thought was perhaps your repro head was interfering with the playback head, but even at 15ips, you're looking at a distance of .03 inches (15 inches/second * .002 seconds per cycle = .03 inches per cycle), which is much much less than the distance between your record and playback heads would be (probably about 3/4" on a machine like that, which would come closer to roughly 50 milliseconds, or a fundamental frequency in your comb filtering of about 20Hz).
Anyway, that last paragraph is mostly just me thinking through the math of it all, but yes, check your measurement setup, try one channel at a time, after doing a full erase pass on the section you're recording just to make sure you're not doing anything funky.
Also, isn't this a 4 track machine? Make sure you're erasing all four tracks, you could be having some kind of bleed through it as well.
Hope that helps!
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it was actually cracked ferrite next to the main gap. Changes the head and problem solved. Thank you!
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Huh, probably causing a weird harmonic "ring." Glad you found it!
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Thank you for response. That was very informative. I’ll research this comb filtering effect. I also will update you once I find the culprit.
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I’m trying to locate the problem with Akai GX-650D.
I checked playback FR and it seem to be ok. Alas I see that recording FR is weird on left channel. Does anybody has some ideas about the causes? Thanks
PS Left VU repeats the wavy pattern.
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When was the last time you cleaned all of the pots/switches? The record switch may be dirty.
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thanks! can bad contact in switch cause repeating pattern of output level in increasing frequency range?
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I'm not very knowledgeable on the technical side, I'm sorry, but I can say that in my experience with bad connections and shorted cables, the high frequency seems to start dropping out first.
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anyways! thank you! I’ll definitely check all the related switches :)
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That frequency plot looks a lot like comb filtering, which is generally caused by phase cancellation. Not sure what would be causing it, though.
Out of interest, did you record both channels at once? Do you still get the same graph if you record only the left channel and record silence on the right one? Just in case there's some kind of crosstalk going on?
ha! that’s very interesting suggestion. I’ll try to record separately today and come back with the results!
This is above my pay grade but I'd still be interested to know if it makes any difference. Self-oscillation in the amplifier is another possibility but whether it would result in this kind of thing I don't know.
Since one channel works and the other one doesn't, one possibility might be to study the schematics for the record amplifier and inspect each component in the signal chain with an oscilloscope while it's recording a test tone at one of the problem frequencies, see if you could find what point the two circuits behave differently. But that's only something you'd try if they're definitely not interacting (i.e if recording just the left channel still comes out garbled), and it does require some knowledge of electronics repair.
Good luck!
much appreciated! at the moment I only have multimeter, but I’m planning to go through signal path in rec. board to find the culprit. But looking forward I’ll change all electrolytes and TRs anyway.