Lousy Sirius reception on the 300C

The satellite signal is literally line of sight. If you could "see" the satellite like a star in the sky, anywhere your antenna can not "see" the star there is no reception. That's why there is no reception in tunnels, under the awning at the gas station, in your garage or carport, etc. That's the only downside I've found regarding satellite radio, so far.

I read somewhere that there is supposed to be 10 seconds of buffering built into the Chrysler/Sirius radios. So a disruption of less than 10 seconds, like passing under an underpass on the interstate, would not disrupt the audio output. But my personal experience is that I lose the audio every time I go under an underpass, no matter how small it is or how fast I'm moving. I would say my radio has no buffering at all.

For that reason, I would expect a canopy of trees to disrupt the signal. I bet the reception is better under those trees in the winter when there aren't any leaves.

Dan

Buffering can't help a radio signal because the information is only broadcast once, if it's lost it's lost. The reason buffering works for CDs is because it's possible to go back and re-read the missed information. The CD reader is much faster then it needs to be to play an audio CD, think about the CDROM in your computer, it's probably 48X which means that it reads at 48 time the speed that is needed to play audio. The CD player in the car is just a CDROM reader so it can get way ahead, if the car hits a bump and the head jumps it has plenty of time to reposition it before the buffer runs dry. Obviously that can't work for radio unless they broadcast the stream at least twice (simple error correction only works for a few bits, it can't compensate for seconds worth of lost data). Sirius would never choose to duplicate the data streams, if they had the bandwidth for that they would just broadcast twice as many channels.

Assuming that they broadcast, say, 1 second's worth of audio in 0.9 seconds, and use the other .1 second to broadcast some future segment of audio that's coming up in the next minute or so.

This review article:

indicates in several places that the authors believe there is such a buffer:

----------------- Since the XM satellites are positioned over the equator, the signal hits North America at a lower angle than the one from the Sirius birds, so various stretches of the southern canyon wall blocked the XM signal just long enough for the receiver's memory buffer to run dry. In all, XM muted very briefly four times during the half hour or so it took us to drive through there. The Sirius system deftly handled the twisting curves, dropping out only once when both systems lost their signals for about 3 seconds as we drove through a 50-yard tunnel blasted through solid rock.

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I guess we also learn that Sirius should have better signal characteristics than XM because of satellite placement.

----------------- We were surprised when the XM radio's buffer couldn't handle some heavy foliage, which caused the signal to briefly drop out three times. The Sirius system, on the other hand, never dropped out as we drove through the high country. To give the systems a final torture test, we returned to Denver via Vrain Canyon Road, another twister with looming granite walls. XM briefly muted twice, and Sirius muted once. In all, during the 12 or so hours of our two-day drive, XM muted

11 times and Sirius muted 5 times.

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This page is more specific about buffering:

---------------- My XM receiver has a buffer to prevent signal loss in short tunnels and underpasses or when passing the north side of tall buildings. The satellite broadcasts each station?s signal twice, with a four-second delay between them. The first signal goes into memory while the second is played live.

Whenever the second signal is interrupted, the XM receiver seamlessly switches to the signal in memory. Since it takes less than four seconds to pass through most underpasses, even at city speeds, there are almost never any breaks in what you hear. The only times I lost service were when I parked nose-in on the north side of a tall building or when I went into some garage buildings.

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However, the author (above) may be confused about how that buffer operates, as indicated in the following:

---------------------- A four-second diversity buffer is integrated into the system. - Ensures continuous reception during shortblockages; e.g., traveling under an overpass. - Processes signals obtained from satellites and a terrestrial network of transmitters

---------------------

A "diversity buffer" could mean a buffer filled from a "diversity" of sources (ie terrestrial helping to keep the buffer full).

Slightly off topic:

How does satellite radio work from a billing and radio-activation point of view?

Is there a card or key or something that is inserted into the radio to activate it? Are activation codes broadcast over the air and the radio looks for it's own serial number and if it doesn't find it - it won't play?

Has anyone hacked a radio to get it to play?

Where I live, the same rural areas that are awful for cell phone reception are also awful for my XM reception, but of course for a different reason (cell tower location vs. tree canopy). There are certain stretches of road that I drive fairly regularly in which I get no or extremely choppy reception for miles at a time (no better or worse than trying to pick up a distant AM or FM station that is located back in the "civilized" world, and when the XM does come in, of course it is crystal clear except for the momentatry dropouts). Also, on East/West bound 4-lane state hiways where there is a wide clear view of the sky but tall trees to the south, I lose the signal for a second or less at a time which I'm realizing after Mopar Man's post is because the satellite is over the equator. I don't get dropouts on interstates because of the much greater tree setback.

We need to start a campaign to cut down all the trees to make our world a better place. 8^)

Bill Putney (To reply by e-mail, replace the last letter of the alphabet in my adddress with the letter 'x')

This one mentions that XM broadcasts the signal twice, it says nothing about Sirius broadcasting twice. I suspect that Sirius doesn't. The first article mentions terrestrial gap fillers, I'm guessing that there are none of those in my area. Both reviewers got better behavior on Sirius then I'm getting, but they did their tests in the West. I'm in Massachusetts, I'd like to know how well Sirius works for other East Coast users.

The first article mentions that Sirius uses a mediocre codec, but claims that Sirius was improving it. I have to agree that the sound quality is slightly disappointing. On the classical music stations the quality is decent but it's not nearly up to CD standards. I'd also say it's slightly inferior to FM, WCRB and WGBH-FM sound richer. The NPR channels are much worse then FM NPR, it's basically AM quality. I'm guessing that they think of NPR as talk radio and most other Talk radio is AM so they've only allocated enough bandwidth to meet AM quality standards.

Point well taken. Obviously, you are correct. There can be no buffer unless the signal is repeated somehow. I can't remember where I read the article that mentioned buffering.

Dan

Dan wrote:

A particular program that I listen to on XM also happens to be live broadcast on a local "traditional" AM station. I have flipped back and forth to compare sound quality, and there is a definite and consistent second or two delay on the XM broadcast. I have always assumed that they did what General Schantzkoph suggested as a possibility, that is the XM signal could be being broadcast twice, the earlier signal going into a constantly rotating circular buffer with the buffer size being exactly equal to what it would take to hold the information being broadcast thru the delay period. As GS said, it could double the bandwidth required, except that the backup (advance buffered) signal could be greatly reduced in bandwidth for an acceptable level of overhead bandwidth. Degraded quality in occasional bursts would not be percieved like it would if the main signal were of the same low bandwidth, and certainly would be better than the dropouts that would otherwise occur. Not saying that they *are* buffering it - I just assumed they were due to the delay that I was hearing in the XM signal compared to the local station broadcast of the same material. There could easily be other credible explanations for the delay (such as the signal travel time to and from the satellite. Come to think of it, that's probably the entire explanation (same exact real latency you get with a satellite internet connection compared to, say, DSL).

Bill Putney (To reply by e-mail, replace the last letter of the alphabet in my adddress with the letter 'x')

You made an interesting observation that the backup channel could have much greater compression then the primary channel because it would only be used as a gap filler, that didn't occur to me. Using 10% of the available bandwidth to reduce the drop outs would make perfect sense, I had been assuming that it would have to be 50% but I was thinking about data not audio. One of the articles that mopar man pointed to said that XM did broadcast the signal twice, there was no mention of Sirius doing the same.