high quality audio on VHS tape part 3

Описание: For more techn. info about HiFi audio on VHS

see part 1 of this serie;    • VHS modified as high quality audio recorde...  
Freq asked questions at;    • VHS modified as high quality audio recorde...  

In this part 3 of the series an introduction and description of a
UNIVERSAL AUDIO ENCODER FOR VHS.

The first part of the video a general description and then an explanation for amateurs and technicians who may want to build something themselves. Note; that an oscilloscope and (preferably) spectrum analyzer can be useful.
The necessary experience is also necessary.

General information.

The main difference between open reel to reel and VHS HiFi is the limitation by reel to reel. By using (reel to reel) the hole dynamic range of the tape is used – because it’s analogue - until the loud portions of the audio are saturating the tape and therefor get distorted. (by the best bias settings) So the very quite portions (audio) are dangerously close to the noise floor. Only thing that helps, is compression and / or pre emphasis. Something the analogue lovers do not want. Yes or no compression (DBX 1) is another discussion.

VHS HiFi das not have that problem, because it’s FM modulated on a carrier(s). So the record current to the (video) heads on the rotating drum, can be (beyond) the point of saturation of the tape. By play-back, it is again heavenly limited (clipped) before going to the FM demodulation. There is no problem of linearity or distortion by saturated tape. But FM modulation comes with a little noise. (also look at your radio broadcast transmitters) For that purpose they use pre-emphasis. In Europe 50usec and in the States 75usec. (6db/octave) To get even more dynamic range, SOME use compression / decompression. BUT every brand has it’s own ideas in what to use for that extra 10DB S/N. So there is little compatibility between the brands.

In short VHS HiFi is a poor man’s studio recorder , especially in the first recording generation very good. But it is not a mastering machine.

Reel to reel is much simpler to deal with and used with a pro compression / decompression adequate for high end recordings. Disadvantage – if you want quality, it will cost you a lot of money if you can find a perfect refurbished unit anyhow. And good tapes, difficult to find!

VHS HiFi is complicated and the machine itself not usable without modifications. Video part must not be used, switch over point (video drum) has to be carefully adjusted. Not used heads in the rotating drum must be shorted. Video heads must be used for audio recording (because they have the widest cap). Drop out detection must be switched off. Record current must be optimized for the used tape(s). Preferable the recorder that was used for recording, must also be used for play back. The already build in electronics for HiFi audio CAN BE USED . Manual volume control is preferable for quality recordings. Already available on the machine or modified.

The new VHS machines usually use large scale integrated circuits . Especially the audio / HiFi section. These are internally controlled and adjusted, often I2C. To intervene in the operation of the IC's - for example manual volume control - you have to apply these signals. They have to be generated externally, a nice job for the digital wizards among us. I myself found it easier to build new circuits. So an older machine (or pro one), is simpler to modify than an newer consumer one. Or you have to build your own circuit board and insert that in the VHS of choice.

For now I am designing an external unit , that can be used on any VHS machine be simply connect it to the video input. NOTE it will be not for sale! I am an tech. engineer, not a salesman.

Due to the high speed of the video heads , wow and flutter are extremely low. Also the audio bandwidth can be expanded to 30kc without any problems. See video 1. The used frequencies (1.3 and 1.7MHz USA) and the frequency deviation for the audio carriers, are existing frequencies. When switching off the video part, the full space of 5MHz becomes available. But when using two channels, a low frequency is preferable (2MHz). You are free in selecting your carriers and freq. deviation. The use of pre emphasis and compression, is also free. BUT without these, the specifications will be less than publicized in my video. Especially the S/N ratio will drop (to 54dB?).