The old Dolby analog noise reduction scheme combined three of your favorite topics:
- envelope tracking
- compression (well, actually dynamic scaling)
- filtering
The idea was to use envelope tracking to allocate a portion for high frequencies (which tended to have lower power in the source as well as more hiss in the media), then use a vari…
The old Dolby analog noise reduction scheme combined three of your favorite topics:
- envelope tracking
- compression (well, actually dynamic scaling)
- filtering
The idea was to use envelope tracking to allocate a portion for high frequencies (which tended to have lower power in the source as well as more hiss in the media), then use a variable amplifier (with one of those newfangled FET things acting as a variable resistor) to opportunistically increase the amount of gain and the filter turning point on the high pass.
The real trick was to correctly match the envelope tracking on the replay so the filter emphasis could be correctly reversed. That was never perfect, because the initial compression was based on the input signal and the recipient had to reconstruct the input only after knowing how to reverse the compression, but in practice Dolby found a scheme that converged well enough to satisfy listeners, especially as an alternative to the hiss of standard tape. Over time later schemes got better, and these days the licensing revenues built Dolby into a large business that is in video as well as audio.
This is why I love writing on the internet. Never in a million years would I have put those three things so nicely into a piece of history. Thank you for sharing!
The old Dolby analog noise reduction scheme combined three of your favorite topics:
- envelope tracking
- compression (well, actually dynamic scaling)
- filtering
The idea was to use envelope tracking to allocate a portion for high frequencies (which tended to have lower power in the source as well as more hiss in the media), then use a variable amplifier (with one of those newfangled FET things acting as a variable resistor) to opportunistically increase the amount of gain and the filter turning point on the high pass.
The real trick was to correctly match the envelope tracking on the replay so the filter emphasis could be correctly reversed. That was never perfect, because the initial compression was based on the input signal and the recipient had to reconstruct the input only after knowing how to reverse the compression, but in practice Dolby found a scheme that converged well enough to satisfy listeners, especially as an alternative to the hiss of standard tape. Over time later schemes got better, and these days the licensing revenues built Dolby into a large business that is in video as well as audio.
This is why I love writing on the internet. Never in a million years would I have put those three things so nicely into a piece of history. Thank you for sharing!