Today I ran a few audio tests on an AG-1980. They included recording a 1 kHz tone, some white noise, and a sine wave sweep from 1K to 18 kHz. These were recorded to both the HiFi and Linear tracks on a freas (if old) Memorex HS VHS tape. I than played the tape in the same machine and captured playback with a M-Audio Delta-66 sound card and Audition 3.0. The attached PDF file shows several wave forms and spectral displays from the capture. Why only 18 kHz - because only dogs and 16 year olds can hear above that.
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Conclusions
- The AG-1980 provided a reasonably flat linear track in self-record mode, down about 3 dB at 10 kHz and after that it falls rapidly, down 16 dB by 12.7 kHz. However the HiFi track was flat out beyond 18 kHz.
- The AG-1980 linear track output appears to actually have a bit of a "notch" filter, probably to suppress audio around the horizontal frequency. This is evident in the spectral display and the frequency analysis display, where the noise floor rises to a shelf level above 15.7 kHz. The HiFi track has a consistent noise level (no notches)
- Harmonics (especially 2nd and 3rd) are evident in both the linear track, but to a much lesser extent in the HiFi track. (Probably around 1% distortion.)
- The Spectral frequency display shows that the linear track contains some sweep audio out to 18KHz, and its harmonics to the limit of the sampling rate (48 kHz). I do not know whether these are bleed or real, but the overall levels are low.
- The linear track white noise recording level show clears shelving above the "notch" frequency.
Interestingly, the JVC HR-S3500U I have does not have the "notch" filter on its linear track's output frequency response. Further, the Sweep output from the AG-1980 tape shows marked indication that its audio head azimuth did not matching that of the AG-1980. The effect on the captured waveforms is dramatic. (The alignment issue was verified by a self-record test with the sweep.) The JVC linear track self-record test also did exhibited aliasing, horizontal frequency bleed, and other issues that made the output look like garbage on a technical display, although much of that is probably not audible to most listeners.
Bottom line on this is one can not trust linear track head azimuth alignment when moving from one machine to another, especially if the machines have not been aligned recently to the same standard.
This brings to mind the possibility that the HiFi track on the First Gen might sound better because is is not tainted by potential linear head azimuth alignment variation compared to the master tape's recorder.