So I recently got a new Ford Ecoboost Mustang. And with the purchase came a 6 month subscription to Sirius/XM. I know. I know. Compressed audio sounds _________ (insert your preferred adjective). However, it sounds much, much better than it did several years ago. I recall hearing my sister's XM radio back in the day, and the compression error in the HF made it unlistenable. That is no longer the case, though there is still compression error in the HF. But much, much less.
I normally listen to music fairly loud. But today I had it turned down to background music SPL. And I could easily hear the compression error in the HF. Then I cranked it up! And at high volume, I could no longer detect the error. So I find myself questioning what is going on here? Perhaps it has to do with the sensitivity of human hearing at low volume? Or perhaps I am totally nuts! Either way, I have enjoyed a summer of listening to the Beach Boys and my favorite 80's hair metal. And when I get tired of music, I flip over to the comedy stations to get a good chuckle. What's not to like about that?
Post by Michael Lawrence on Aug 25, 2018 14:47:53 GMT
Welcome to the forum. I agree with you that the SiriusXM encoding can be a little heavy-handed at times. It tends to be lower-level spoken word content that really sounds the worst - like everything on the comedy stations. I suspect a lot of the source recordings of those comedy specials have already been lossily compressed once so it's likely compounding the issue.
A lot of the webpages I found claiming to have technical information on S/XM sound quality were wildly inaccurate, but a few reputable sources say that the Sirius audio is originally stored in MPEG-I Layer II format at 384 kbps and then compressed with a proprietary version of Lucent's perceptual audio coder algorithm for transmission. So actually my guess about two stages of compression seems to be dead on. "Perceptual audio" means lossy compression, such as MP3. Apparently XM audio is built from variable numbers of channel bitstreams depending on the content and can be anywhere from 4 kbps to 64 kbps. So, yes, it doesn't sound great.
Thanks for the reply. It is definitely that variable part of the encoding that is more bothersome as this causes "chatter" that is generally quite annoying. I've often thought that they would be better off to band limit to around 10kHz to minimize some of the bandwidth required and then use a constant bitrate encoding scheme. But this is probably an oversimplification of the problem.
The reason is bitrate. The two satellites use different systems, but both have around 4MHz of bandwidth to share amongst ALL the channels broadcast. This means that each music channel winds up getting enough bandwidth to transmit around 48 - 64kb/second of data. The encoding scheme is a modified AAC algorithm somewhat better than MP3, so this can get you about the sonic equivalent of a 96k MP3.
Channel quality was in one of two flavors, stereo music channels at 39 kbit/s and mono talk channels at 16 kbit/s using proprietary compression. Many subscribers have complained about the low quality of satellite radio sound. But providers have stuck with the plan for more channels instead of better quality. HD terrestrial digital radio, a competitor has always used this difference as a selling point.
So how badly is it compressed? How about 32 kbps (kilo bits per second). Let me put that into perspective for you. When you buy a CD in the store (I think they still exist) the CD relays 1411 kbps of information. So let’s do the math.
Okay, so 32 divided by 1411 equals about .02 or we can call it 2%. You’re listening to 2% of the original information.
And yes, your ears pick up HFs more easily when the SPL is high. Same goes for bass. Normally I mix around 66dBC with my mixcube (90Hz-10kHz) but when I do my highs and lows, I crank it to 3 o'clock, about +7dB, for more perception. That's roughly right for my room size and subjectively it's correct also. Actually, I occasionally crank it to check that the midrange isn't piercing. I've been doing that recently since I got blasted for harsh mids, anyway.