[Geluso]

Can anybody tell me the frequency response of XM broadcasts. It is
general knowledge that the frequency response of FM broadcasts is
basically 30Hz-20KHz. In contrast, despite quite a bit of research,
I have been unable to locate any mention of XM's frequency response
whatsoever.

I would like to know XM's frequency response because I just bought a
Polk XRt12 Home XM Tuner, which is suppose to be one of the best XM
receivers available, yet I am extremely disappointed in its sound
quality. In addition to very poor stereo separation, the music is
completely washed out. Brilliancy is non-existent. The highs are
dull, and the midrange is muted. Even the human voices are muffled.

True, I have heard many negative comments regarding XM's compressed
signal. But I have also heard from numerous sources that XM
delivers near CD-quality sound. It is this significant discrepancy
of opinion that has led me to seek a quantifying answer to XM's
frequency response.

The very poor sound quality that I am getting out of my Polk
receiver puzzles me, because when I had the opportunity to listen to
XM's programming through a DirecTv system, the sound quality was
superb. Does anyone have an explanation for this great difference?

Your answers to these questions will be more than appreciate.

Bill Geluso

[---MIKE---]

Your frequency response for FM is incorrect. Generally it is assumed to
be 50 to 15000 HZ. There has to be a sharp cutoff above 15000 to remove
the stereo decoding signal.

---MIKE---
In the White Mountains of New Hampshire
(44° 15' N - Elevation 1580')

[[email protected]]

Geluso wrote:
Can anybody tell me the frequency response of XM broadcasts. It is
general knowledge that the frequency response of FM broadcasts is
basically 30Hz-20KHz. In contrast, despite quite a bit of research,

Not stereo FM, it's not. Stereo is limited to a maximum high-
frequency cutoff of 15 kHz.

[Peter Wieck]

Geluso wrote:

Can anybody tell me the frequency response of XM broadcasts. It is
general knowledge that the frequency response of FM broadcasts is
basically 30Hz-20KHz. In contrast, despite quite a bit of research,
I have been unable to locate any mention of XM's frequency response
whatsoever.

The standard FM bandwidth is: +/-0.5 dB, 30 Hz to 15 kHz.

"Audio channels on XM are digitally compressed using the aacPlus codec
from Coding Technologies for most channels, and the AMBE codec from
Digital Voice Systems for some voice channels." -- Wikipedia

And, depending on how the system is decoded in your head-end, that
could cut off anywhere around 12Khz or so.... which from what I
understand (purely by anecdote) seems about typical.

The AMBE codec, BTW, is what satellite phones use and is limited to
voice-frequencies only... which makes sense, after all.

Keep in mind that XM (and Sirius) were designed against the automotive
environment. Subtle nuances are not required, and furthermore would
chew up bandwidth... can't have that, can we? Don't expect such a
system to translate well to home audio.

Small Political Rant: Once upon a time, not so very long ago, Radio and
Television were OTA, free, and somewhat varied (at least radio was
such). With the gutting of the FCC, the onset of protected cable
companies and the general impatience of Americans, OTA is largely being
displaced by fee-for-service delivery systems, cable, satellite and
so-forth. Accordingly, the OTA outlets are struggling and
consolidating, and excepting public and college stations, seriously
dumbing down. The AM and FM bands as we know them are threatened (as
well as OTA television) and there is a push to eliminate them
altogether... to be replaced by fee-for-service systems. When one has
to pay for it, Speech ain't nohow free.

Write/call/e-mail your representatives in Washington. The "people" own
the airwaves, not the corporations, so take them back. Put some teeth
back into the FCC.

Peter Wieck
Wyncote, PA

[[email protected]]

Geluso wrote:
Can anybody tell me the frequency response of XM broadcasts. It is
general knowledge that the frequency response of FM broadcasts is
basically 30Hz-20KHz. In contrast, despite quite a bit of research,
I have been unable to locate any mention of XM's frequency response
whatsoever.

I would like to know XM's frequency response because I just bought a
Polk XRt12 Home XM Tuner, which is suppose to be one of the best XM
receivers available, yet I am extremely disappointed in its sound
quality. In addition to very poor stereo separation, the music is
completely washed out. Brilliancy is non-existent. The highs are
dull, and the midrange is muted. Even the human voices are muffled.

True, I have heard many negative comments regarding XM's compressed
signal. But I have also heard from numerous sources that XM
delivers near CD-quality sound. It is this significant discrepancy
of opinion that has led me to seek a quantifying answer to XM's
frequency response.

The very poor sound quality that I am getting out of my Polk
receiver puzzles me, because when I had the opportunity to listen to
XM's programming through a DirecTv system, the sound quality was
superb. Does anyone have an explanation for this great difference?

Your answers to these questions will be more than appreciate.

Bill Geluso

Was your XM tuner and the DirectTV system connected to the same hi fi
system? You could have experienced your XM tuner, having been connected
to a very revealing system, showing up all the flaws in XM
transmissions, whereas the system that the DirectTv was connected
wasn't as highly resolute.

XM transmaissions can be very good, but to take advantage of the
ability to shove more content down the same bandwith, and hence save
some money, XM has sacrificed quality of the content, significantly.

CD

[[email protected]]

Geluso wrote:
Can anybody tell me the frequency response of XM broadcasts. It is
general knowledge that the frequency response of FM broadcasts is
basically 30Hz-20KHz.

I think it's less than that. You could probably find more info by
searching the web or doing a little library research.

In contrast, despite quite a bit of research,
I have been unable to locate any mention of XM's frequency response
whatsoever.

See above.

You might also search the web and newsgroups (Google has that option)
for newsgroups devoted to XM and Sirius where you could find people who
might better answer your questions.

You could also try:

rec.audio.tech

I would like to know XM's frequency response because I just bought a
Polk XRt12 Home XM Tuner, which is suppose to be one of the best XM
receivers available, yet I am extremely disappointed in its sound
quality. In addition to very poor stereo separation, the music is
completely washed out. Brilliancy is non-existent. The highs are
dull, and the midrange is muted. Even the human voices are muffled.

(snip)

The very poor sound quality that I am getting out of my Polk
receiver puzzles me, because when I had the opportunity to listen to
XM's programming through a DirecTv system, the sound quality was
superb. Does anyone have an explanation for this great difference?

I'm just guessing, but the symptoms you describe (such as "muffled
voices") sound like maybe something in the Polk, in your cables, or in
your audio system is wired slightly incorrectly, making the audio
signal from your Polk XM tuner out of phase.

If the above is correct, I'd start by connecting the Polk tuner with
your system with another set of audio cables. If that doesn't fix the
problem, try connecting the Polk to your preamp or receiver at
different inputs.

You could also take the Polk to a friend's house and trying it there.
Maybe the best idea of all is to take the Polk back to your local
dealer and getting the dealer to start sorting out the problem.

How strong is the signal from the XM antenna? Could that be the
problem?

Your answers to these questions will be more than appreciate.

Bill Geluso

[Walt]

Geluso wrote:

Can anybody tell me the frequency response of XM broadcasts. It is
general knowledge that the frequency response of FM broadcasts is
basically 30Hz-20KHz.

The spec for FM calls for response out to 15khz, with a fairly steep
rolloff above that. The stereo pilot tone is at 19khz, and for that to
work there can't be any audio signal that high.

That said, the sound quality issues with FM are not due to inadequate
response above 15k. For one thing, very few people can hear above 15k.

In contrast, despite quite a bit of research,
I have been unable to locate any mention of XM's frequency response
whatsoever.

I'm not sure. But again, the frequency response is not the issue. See
below.


I would like to know XM's frequency response because I just bought a
Polk XRt12 Home XM Tuner, which is suppose to be one of the best XM
receivers available, yet I am extremely disappointed in its sound
quality. In addition to very poor stereo separation, the music is
completely washed out. Brilliancy is non-existent. The highs are
dull, and the midrange is muted. Even the human voices are muffled.

Welcome to the world of digital audio compression. You describe the
artifacts quite well. You might add that the soundstage is flat,
two-dimensional with no depth or sense of space. That's what happens
when you throw away all those bits that you supposedly can't hear.

True, I have heard many negative comments regarding XM's compressed
signal. But I have also heard from numerous sources that XM
delivers near CD-quality sound.

Don't beleive the hype. XM isn't CD quality. Not even close.

It is this significant discrepancy
of opinion that has led me to seek a quantifying answer to XM's
frequency response.

The answers lie in a) what bit rate are they using (64k? 128k? ) and
b)what compression algorithm are they using.


The very poor sound quality that I am getting out of my Polk
receiver puzzles me, because when I had the opportunity to listen to
XM's programming through a DirecTv system, the sound quality was
superb. Does anyone have an explanation for this great difference?

Perhaps it was a different bit rate on the other system? Or the
material was chosen to minimize the compression artifacts?

//Walt

[bob]

Peter Wieck wrote:

"Audio channels on XM are digitally compressed using the aacPlus codec
from Coding Technologies for most channels, and the AMBE codec from
Digital Voice Systems for some voice channels." -- Wikipedia

And to rattle your cage a little more, I think the rate is 48kbps.
(Which, when you think about it, tells you just how good AACPlus really
is.)

And, depending on how the system is decoded in your head-end, that
could cut off anywhere around 12Khz or so.... which from what I
understand (purely by anecdote) seems about typical.

Part of the way AACPlus works, AIUI, is that it strips out *everything*
above a certain frequency (I think 7kHz), and replaces it with a bit of
code, essential an algorithm that reconstructs the high frequencies
from what's still there. IOW, all you're getting up high is a rough
approximation. Again, it's amazing that it sounds as good as it does.

The AMBE codec, BTW, is what satellite phones use and is limited to
voice-frequencies only... which makes sense, after all.

Keep in mind that XM (and Sirius) were designed against the automotive
environment. Subtle nuances are not required, and furthermore would
chew up bandwidth... can't have that, can we? Don't expect such a
system to translate well to home audio.

Small Political Rant: Once upon a time, not so very long ago, Radio and
Television were OTA, free, and somewhat varied (at least radio was
such). With the gutting of the FCC, the onset of protected cable
companies and the general impatience of Americans, OTA is largely being
displaced by fee-for-service delivery systems, cable, satellite and
so-forth. Accordingly, the OTA outlets are struggling and
consolidating, and excepting public and college stations, seriously
dumbing down. The AM and FM bands as we know them are threatened (as
well as OTA television) and there is a push to eliminate them
altogether... to be replaced by fee-for-service systems. When one has
to pay for it, Speech ain't nohow free.

Write/call/e-mail your representatives in Washington. The "people" own
the airwaves, not the corporations, so take them back. Put some teeth
back into the FCC.

Amen, brother. I live in a major metro area where I can get classical,
jazz, and college stations--sorta. Lately I've been relying more on
their Internet feeds, some of which aren't even as good as XM.

bob

[NoDownTime]

Frequency response isn't the issue.

While XM's sound quality is better than the other company's, it is not
CD quality (and they don't advertise it as such anymore). It was
closer to CD quality when they started out but as they've moved up to
170 channels, the bitrates have declined on the channels. On March 1,
they added a lot of music channels and SQ suffered.

While you'll probably never heard CD quality from satellite radio, in
XM's case the current SQ levels are probably temporary -- they will
have additional throughput soon, and many anticipate they will go back
to the higher quality streams on most channels.

The best SQ on XM is heard on XM 76 (Fine Tuning) and XM 113 (Normally
popular classical, but right now, Christmas music). You can use these
two channels to gauge the best you can expect for the future, I would
think.

XM's SQ is quite a bit better than Sirius', particularly when it comes
to stereo separation and dynamics. But it still isn't close to a
128kbps MP3.

It is a tradeoff -- sound quality vs. diversity of content. For me, it
is an okay one -- when I want sound quality, I can get it through my
own collection -- but there is so much stuff available on XM that you
simply could never have in a private collection, I find it worthwhile
to sacrifice quality most of the time to get the additional content
which I'd never hear otherwise.

The Polk receiver is a good one, and it is fair to assume that for now,
you're getting the best SQ satellite radio has to offer. I'm hoping
to hear something about SQ improvements in January.

[NoDownTime]

The very poor sound quality that I am getting out of my Polk
receiver puzzles me, because when I had the opportunity to listen to
XM's programming through a DirecTv system, the sound quality was
superb. Does anyone have an explanation for this great difference?


Sorry, I failed to address this -- you are exactly right here --
through the DirecTV system you are getting a much higher bitrate than
over XM's satellites.

XM broadcasts via its own satellites using AACPlus at bitrates from
32-64 kbps (for music channels, with the actual rate depending on the
channel). When you receive the signal via DirecTV, the compression is
much less -- I believe at 128kbps or higher. So, you are exactly right
about this -- if you have an option for using the DirecTV signal, it is
probably a good idea.

You can get 64kbps via streaming (this is likely to increase next year,
so I'm hearing, to 128kspb). I do think frequently, streaming from my
laptop/Itunes to my sound system, and it works pretty well. Also,
Sirius does offer 128kbps streaming right now for an additional fee. I
think it is likely XM will offer the same in the coming months.