All-pass filter: Should I go digital?

[Electroshark]

I'm currently building a system which needs an all-pass filter with a delay as constant as possible over a ~200 kHz frequency range (i.e. this range can be anywhere in between 100 Hz to 500 kHz). To complicate things even more, I need the delay to be voltage adjustable. I assume that it's very difficult to build such an elaborate analog filter. Am I right? If so, should I consider building a digital all-pass filter instead? Are modern audio delay circuits using these types of filters? Do they exist in IC?

Any advice would be greatly appreciated...

 

[Nigel Goodwin]

Decades ago audio delay circuits (echo, flangers etc.) used BBD's (Bucket Brigade Devices), however these have been obselete for a great many years now. More recent methods use digital memory, converting the audio to a digital value, reading it into a memory and reading it back out later on. Current technology though would use a DSP to do the job.

However, you're a bit vague about what you're actually trying to do, and it doesn't appear to really be audio related?.

 

[Electroshark]

Yeah... you're right, it's not audio related. Basically, what I want to do is to simulate the propagation of wideband pulses through coax cables of various lengths. Since I’m working with analog signals, I wanted to build an analog all-pass filter to do the job, but I quickly learned that it was very difficult. I think I’m going to go with your advice and consider using a DSP. Do you guys know of any low cost DSPs specialized for audio delays? If not, don’t worry about it, I’m going to look it up…

Also, you seem to be saying that a DSP is better than using a digital memory buffer for an all-pass filter... why is that?

 

[Nigel Goodwin]

I'm not saying it's better, just that DSP's have basically taken over from memory methods.