ETI vocoder anybody ?

[Haible Juergen]

Hi,

has anybody built, or does anybody own an ETI vocoder 
(= elrad vocoder, = Powertran vocoder) ?

I have a particular question about the slew / freeze function.

It's done with an electronic switch that is pulse width modulated,
to change the time constant of a simple RC lowpass filter.
When the PW is 0, it acts as S&H; that's the freeze mode.
(I assume the EMS vocoders do it a similar way, but I have never
seen schemos for EMS vocoders.)

Now, the problem is that the frequency of the PWM signal is *not*
above the audio range, but at 1kHz. 
Why not a higher frequency ? They need precise PW down to 
0.5%, and that's 5us @ 1kHz. (If the frequency were 50kHz, that
would be 100ns.)

The question is a simple one: Does anybody have practical
experience with the ETI vocoder ? Can you tell me how much of the
1kHz switching signal is audible at the output ? Is  it audible
at all ? (The 1kHz  would probably not be present with minimum slew rate,
or in freeze mode, but in any setting in between.)

JH.