VHS mono audio track auto-filtering?
 

I've learned that some VCRs have this annoying feature that I'd call "auto mono audio bandwith filter", in which they decide which frequencies are to be mitigated (based on tape quality, I guess), resulting in a rather lackluster mono audio quality. Furthermore, that filtering is kinda random, switching on and off on the same recording from the same VCR.

Do S-VHS VCRs have this annoying audio feature as well or can it be switched off like "edit mode off" for video? If so, does a S-VHS unit like the JVC HR-S7611 enable playing mono audio "as is"? The VCRshop has that model for sale.

What I mean is the MONO track audio behaviour as shown in this spectral view image:

Attachment 16324

Notice how the audio suddenly loses amplification and the 5kHz band surroundings are almost cut. This dramatically changes the audio. The VCR does this on its own. What's more, if I go to that bad part of the tape, the video may or may not provide a good audio response. It's almost random. I was wondering if there are any VCRs that can disable this automatic audio filtering.

EDIT: The audio response shown in the spectral view image is from the same mono track. There is no HIFI track whatsoever. The VCR itself cannot playback HIFI as well.

Some VCRs did Dolby linear sound, the documentation should address it.

SP linear track audio might approach 10 kHz on a good day, but EP/SLP speed is probably going to pass out at around 4 kHz.

Is this happening with commercial tapes or home recordings?
What tape seed?
Is it on one continuous recording, or segments recorded at different times or on different machines?

Can you post a short sample of the sound only?

The tape is a 30 year old BASF 195min PAL tape. The recording is also 30 years old from live TV.

The VCR being used to play these recordings is a Sony SLV-135 (PAL).

The tape contains different recordings from the same (now defunct) VCR, an old National, but they were recording over two weeks.

The audio sample shows what the image also showed, a change in audio response during the same recording.
Attachment 16325

Below is an RX graph of your audio sample. As can be seen, the deep null starts at a higher frequency and gradually settles around 5 kHz, with frequencies above that also reduced but not as much as at 5 kHz.

I'm not sure whether this problem is on the tape, or is caused by the VCR, although it can sometimes be a bit of both. dpalomaki mentions Dolby in this regard, but Dolby was not very common on consumer machines, especially later models.

Does the effect always occur at the same point in a tape or does it occur randomly?

It occurs randomly. Just today, after cleaning the mono track head and initiating the recording session right after turning on the VCR, I managed to get great audio quality without filtering from the part that two days ago I just couldn't get it right. But then, later on day, the issue returned again. I tried cleaning the mono track once more but to avail. I'm beginning to wonder if the VCR temperature is the cause...

Tomorrow I'm trying the "cold start" recording again and hope for the best.