Right ! Exactly Right !
This is why we need to push our favorite vendors to use at least 32
bits. At work - where I don't have to pay for the equipment, Hi Hi, - we
use 64 bit stuff. It's outstanding compared to what has been
traditionally affordable in the Ham world.
However, prices have been going down rapidly as DSP gains
popularity. I hope we can afford 32 bits now.
73 de Gary, AA2IZ
----- Original Message -----
From: "Robert & Linda McGraw K4TAX" <RMcGraw@Blomand.Net>
To: <tentec@contesting.com>
Sent: Monday, December 30, 2002 10:11 AM
Subject: [TenTec] Jupiter Audio vs. DSP
> Reading recently about concerns regarding some distortion artifacts in
> receive audio and in the suggestion that the attenuator should be used
> brought me to disclose a few facts with regard to DSP based receivers.
>
> As with any digital processing system there is a specific dynamic range
that
> the system will handle. It's simply a mathematical relationship. Once
the
> signal exceeds the dynamic range, unlike analog circuits which have a
broad
> or round knee into the distortion range, the digital system goes suddenly
to
> overload mode, thus to a sudden and high level of distortion. In general,
> it sounds "ugly". In the case of DSP for ham receivers, regardless of IF
> DSP or AF DSP the same factor applies. While we would like to think
> otherwise, designers of circuits have yet to produce ACG circuits which
will
> absolutely prevent a wide range of front-end signals from, at some point,
> producing overload of the DSP system. We didn't realize this with
> conventional analog signal chains as the overload or distortion point was
> typically a smooth transition from clean, to very slightly distorted, to
> moderately distorted and finally to total distortion. In the case of
> today's modern receivers and digital signal chains, the attenuator must be
> used to extend the dynamic range of a receiver.
>
> The dynamic range of a digital system is based the number of binary bits
in
> each word. Each bit corresponds to a doubling of the resolution, or
roughly
> 6dB of dynamic range capability. Telephone systems typically use 8 bits
> which is approximately 48 dB dynamic range linear, although effectively
more
> when companded. Today's CD's and most digital recorders use a 16 bit
system
> for some 96 dB or dynamic range.
>
> Look at it this way, making the assumption that 1 S unit is ~6dB, then S-9
+
> 40 dB is 94 dB. Today's receivers can easily hear a signal of -125 dBv
and
> are expected to handle a signal to +40 dBv. This is a dynamic signal
range
> of ~160 dB. Hence the need to apply attenuation ahead of the DSP system
to
> allow it to operate within its limited dynamic processing range.
>
>
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