die@hydra.UUCP (Dave Emery) (10/18/85)
> > >> 5) Worst of all, a distortion problem which seems level and frequency > > >> dependent. This is really apparent with solo recordings (for example, a > > >> female opera singer or a trumpet) where there is no other music to mask > > >> the problem. I would describe the distortion as a harsh, jagged kind of > > >> sound which accompanies the music. This occurs at levels below the red > > >> LED (0dB). I recorded some sine wave test tones at different levels and > > >> frequencies. The performance seems OK at low frequencies, but at high > > >> frequencies the same harsh, jagged sound emerges, somehow superimposed > > >> over the test tone. > > I believe that there is quite simple explanation of this > phenomonon. It has to do with the effect of a slight time delay > error in the signal coming from one spinning head versus the signal coming > from the other. Rereading my own posting by happenstance, another thought occurs to me. The bandwidth of the filters used to split out the specific channel of FM sound (one of four frequencies) may not be wide enough to reproduce heavily modulated signals modulated by high frequency tones especially if a fair degree of preemphasis is used. I do not remember how much preemphasis is actually used with Beta Hifi, but I suspect it increases the deviation of a 10 khz signal by several db over a 1 khz reference. If this is true than a maximum amplitude 10 khz signal (recorded at 0 VU ref) might well modulate the subcarrier to such a swing as to exceed the bandwidth of the filters used to separate it from it's companion signals nearby. And with a modulation index of >4 or 5 (guessing) the sideband energy should spread out quite some additional distance. All of this would cause a very level sensitive distortion with a very steep onset at a recording level threshold near to but possibly not at maximum. The resultant distortion could be very severe if it caused gross phase errors in the sideband components (and particularly falls through the origen as in the near FM threshold click phenomenon). It's quite possible that the choice of FM deviation (recording level) was made to maximize the signal to noise ratio with the full knowlage that the resultant system was incapable of recording full amplitude high frequency signals. MOST music does not contain passages with very high amplitude high frequency notes in the absence of enough energy at lower frequencies to cause considerable gain reduction in the compander (dBx) so perhaps the desire for extreme SNR led to a deliberate overmodulation. David I. Emery Charles River Data Systems 617-626-1102 983 Concord St., Framingham, MA 01701. uucp: decvax!frog!die