Yes, I agree tweaking my deck’s azimuth to match the recording would be the ideal fix. Call me lazy, but the mono mix exhibits conspicuous comb-filtering long before the individual channels do. Moreover, if the HF rolloff were due mainly to the azimuth error, the two channels would still be reasonably matched. I suspect it’s usually a combination of a dirty record head and a poor quality cassette. I note your point that matching my azimuth to the tape would reduce the amount of fiddling I have to do later, and agree with that too.
Koz - Did the above answer your question re: applying an averaged HF correction?
I advised de-skewing the channels near the middle of the tape for a reason: Some recorder/cassette combinations produce a skew that varies from start to finish of a tape side. One suspect here is the tension sensing probe that some low-end decks use to detect end-of-tape, but that’s a guess - I haven’t done any research. My thinking is if the probe is off-center or tilted, or if it pushes the tape against the cassette housing and the housing isn’t perfectly flat in that spot, it strains the tape nonuniformly and can skew the tape in azimuth. Then if the tape tension varies with radius on the supply spindle, the skew will vary.
I should add three points here:
1: Listening to or examining the result after de-skewing in the middle will tell you whether you ought to divide the side into multiple clips and de-skew them individually.
2: If the skew is fluctuating as the supply spindle turns there’s no point in combining the channels at all - you’ll still get that mini-flanging effect throughout, which is more annoying than the hiss.
3: Combining the channels has another benefit besides 3 dB improvement in s/n ratio: Minor tape dropouts that aren’t due to mechanical damage are often not coincident in both channels. A 6 dB dropout is not as bad as a 60 dB dropout, and if you combine the channels after fixing the ones that can be fixed you’ll have fewer of them.
BTW, I’ve found that a single frequency is not the best test signal for tweaking azimuth, even though that’s what standard test tapes have. Sometimes the central peak occurs at slightly different settings in the two channels (maybe head mfg tolerances, who knows?). If I’m using a standard test tape (used to have one, lost track of it years ago) I split the difference with the max amplitude adjustment and then put a scope on the output and matched the phase of the two channels. Back then, I had a basic single-channel scope so I used the Lissajous test: Put one channel on vertical and the other on horizontal and adjust for a diagonal line on the screen sloping up and to the right. If you don’t have a scope, you might have a way to measure the differential voltage across the two channels and null that out. Monoural white noise, however, provides a very sensitive test. When the two channels are combined, the azimuth can be set quickly and very accurately by listening - no need even to meter the output. Besides, it just makes sense to be tweaking out the effect to which the ear is most sensitive, using a test signal that maximizes it. If you prefer instrumentation and have a scope, you can still use the Lissajous test on white noise and you don’t have to search for the global max among multiple peaks.