DSP in receiving systems
 

Hi, all concerned:

Ponderings while listening to a dead 40M c-w frequency ...

What artifacts does latency in a DSP implementation cause?

I'm thinking of eg a "DSP rcvr" OOH and an "analog receiver" OTOH, both
tuned to WWV, and the noticeable delay of the DSP audio from the analog
audio. You know, ti-tick ti-tick instead of tick-tick-tick, and "blurred"
voice announcements.

I've noticed a similar effect in add-on DSP by putting speakers at the input
and at the output of the add-on device. Stereo cans make the effect
downright strange.

Fast c-w QSK would seem to be a challenge, especially if latency varies with
selectivity.

Makes me wonder how the commercial "DSP rigs" perform. I'm mostly
satisfied with an Elecraft K2 (wish it had 200W barefoot, instead of 100W),
and have fond memories of my TS-850's. Never met a commercial radio I
couldn't make better ;o)

IAC, happy Thanksgiving and 73,

Dave, N3HE

"David J Windisch" wrote in message
...
Hi, all concerned:

Ponderings while listening to a dead 40M c-w frequency ...

What artifacts does latency in a DSP implementation cause?

I'm thinking of eg a "DSP rcvr" OOH and an "analog receiver" OTOH, both
tuned to WWV, and the noticeable delay of the DSP audio from the analog
audio. You know, ti-tick ti-tick instead of tick-tick-tick, and "blurred"
voice announcements.

I've noticed a similar effect in add-on DSP by putting speakers at the
input and at the output of the add-on device. Stereo cans make the
effect downright strange.

Fast c-w QSK would seem to be a challenge, especially if latency varies
with selectivity.

Makes me wonder how the commercial "DSP rigs" perform. I'm mostly
satisfied with an Elecraft K2 (wish it had 200W barefoot, instead of
100W), and have fond memories of my TS-850's. Never met a commercial
radio I couldn't make better ;o)

Delay is inherent in most DSP systems used for filtering, because they
generally use a circular buffer for the convolution operation. It acts like
a delay line.

73, Leon

Leon Heller wrote:

"David J Windisch" wrote in message
...

Hi, all concerned:

Ponderings while listening to a dead 40M c-w frequency ...

What artifacts does latency in a DSP implementation cause?

I'm thinking of eg a "DSP rcvr" OOH and an "analog receiver" OTOH, both
tuned to WWV, and the noticeable delay of the DSP audio from the analog
audio. You know, ti-tick ti-tick instead of tick-tick-tick, and "blurred"
voice announcements.

I've noticed a similar effect in add-on DSP by putting speakers at the
input and at the output of the add-on device. Stereo cans make the
effect downright strange.

Fast c-w QSK would seem to be a challenge, especially if latency varies
with selectivity.

Makes me wonder how the commercial "DSP rigs" perform. I'm mostly
satisfied with an Elecraft K2 (wish it had 200W barefoot, instead of
100W), and have fond memories of my TS-850's. Never met a commercial
radio I couldn't make better ;o)


Delay is inherent in most DSP systems used for filtering, because they
generally use a circular buffer for the convolution operation. It acts like
a delay line.

73, Leon


The DSP allows you to implement a type of filter called "finite impulse
response". These have some strong advantages over the type of filters
(infinite impulse response) used in analog systems, mostly sharper
filtering for the amount of ringing, and the capability to make filters
adaptable (like adaptable notching). One price you pay, however, is
latency.

Fast QSK would be interesting, but if you were willing to go without
sidetone and perhaps hear the break request while you're keying I think
it would be doable (I wouldn't know from personal experience -- I do DSP
work professionally but don't have any DSP rigs).

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

What artifacts does latency in a DSP implementation cause?

I'm thinking of eg a "DSP rcvr" OOH and an "analog receiver" OTOH, both
tuned to WWV, and the noticeable delay of the DSP audio from the analog
audio. You know, ti-tick ti-tick instead of tick-tick-tick, and "blurred"
voice announcements.

Seems that it is starting to play role in QSK and contesting. Latest TenTec
Orion seems to be having problem with QSK above 40 wpm.

The other effect is some loss of inteligibility on weak signals. My friend
OK2RZ who started to use TS870 immediately complained about the inteligibility
as compared to straight analog RX. Audio sounded mushy to him, more difficult
to copy weak signals in the noise. To me it was a surprise, but he is appliance
operator with good ears and when he compared two types of rigs, he pointed out
to mushiness of 870 (esp. weak) signals.

We are considering this in the design of Dream Radio One, where heavy computer,
DSP and analog hybrid is coming to play.

Looks like DSP is here to stay, has some advantages, but will need some fine
tuning (no problem, digital stuff, right?).

Yuri, www.K3BU.us
www.computeradio.us

Yuri Blanarovich wrote:

What artifacts does latency in a DSP implementation cause?

I'm thinking of eg a "DSP rcvr" OOH and an "analog receiver" OTOH, both
tuned to WWV, and the noticeable delay of the DSP audio from the analog
audio. You know, ti-tick ti-tick instead of tick-tick-tick, and "blurred"
voice announcements.


Seems that it is starting to play role in QSK and contesting. Latest TenTec
Orion seems to be having problem with QSK above 40 wpm.

The other effect is some loss of inteligibility on weak signals. My friend
OK2RZ who started to use TS870 immediately complained about the inteligibility
as compared to straight analog RX. Audio sounded mushy to him, more difficult
to copy weak signals in the noise. To me it was a surprise, but he is appliance
operator with good ears and when he compared two types of rigs, he pointed out
to mushiness of 870 (esp. weak) signals.

We are considering this in the design of Dream Radio One, where heavy computer,
DSP and analog hybrid is coming to play.

Looks like DSP is here to stay, has some advantages, but will need some fine
tuning (no problem, digital stuff, right?).

Yuri, www.K3BU.us
www.computeradio.us

The "mushiness" sounds like the ADC or DSP data paths aren't deep
enough, so the weak signal is getting lost in quantization noise.
That's unfortunate, so make sure that your "dream radio" has plenty of
"footroom".

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com