[sdiy] Digital echo chip

[jh.]

> ADM is "Adaptive Delta Modulation"... the method used by DeltaLab
> for their digital delays.
>
> The way it works is that the bit referrs to the sign of the incomming
> signal...
> and the sign is integrated and compared with in input.
>
> If the sign remains the same for consecutive clock cycles, the integrator
> current
> is progressively stepped up. So 1 bit in a row is a small step, 2 in a row
is
> a larger
> step... etc.
>
> The technique works very well with a very high clock frequency.


Think of it as similar to Sigma-Delta Converters. But unlike these, not
the actual signal is stored, but the "change of signal": The ADC
estimates the direction (larger / smaller) for the next sample and then
proceeds depending on whether this estimation was right or wrong.
The trick about ADM (adaptive DM) is that the step size for this
estimation is changed depending on the recent bits. In other words, there's
a compander built into the ADC. Sample rate and algorithm that is used for
the compander function can make a huge difference on the quality of ADM.
Take a look at Deltalab's patents to see how they have refined the
algorithm (all implemented in hardware).
This companding function, and the fact that the d/dt of the signal is coded,
leads to the most severe drawback of the method: It's severely
slew limiting ! These delays have an impressive SNR and frequency response,
but *only* for low frequencies and high frequencies with small amplitude.
Not unlike a slow opamp (;->). Fast and large transients is something
they don't like. You get excellent SNR figures, but when you excced the
rise time limit, you get nasty TIM distortion.

JH.