Embedding Data Signals In White Noise 239
Anemophilous Coward writes "ZDNet has the following article which describes a company that 'has devised a method for sending wireless signals over ordinary audio speakers so that humans can't hear them. With this same technology, radio stations can unobtrusively transmit ads, Web site URLs, or information about music and artists to in-car cell phones.'" Here is some further reading about the company, Intrasonics.
For those who don't know what white noise is (Score:5, Informative)
Not based on frequency (Score:5, Informative)
Frequencies are already optimized for human hearing, and it's not usually possible to send, say, a 40,000Hz signal on most anything you can think of, analog or digital. Standard phones have a bandwidth of something like 3 K Hz. CDs of course top out around 20,000Hz, give or take a bit. (It's not a perfect cutoff at 22050, it's a curve, so there isn't quite a point you can say is "the limit".) I don't know for certain but I'd bet FM can't transmit those frequencies and be compliant with FCC regulations. (Of course the tech could do it in theory, but the radio station may have to leave their allocated frequencies to do it; I don't know for certain.) AM could do it in theory but based on the low quality of the signal I hypothesize that something is preventing high frequencies from getting through.
Finally, the coup de grace is that our speakers are optimized for human hearing, pretty much no matter what. Covering the bases from 20Hz - 20,000Hz is a hard enough problem without pushing the required range up another couple of octaves.
In fact, what the company is proposing seems to be in some sense the inverse of MP3 coding. MP3 coding strips the signal of things that you can't hear through by analysing what is psychoacoustically masked [mp3-converter.com] in the original signal. The MP3 encoding process can then focus on just the parts of the signal you do hear, which is obviously going to require less space, except in some pathological cases where the whole sound is perceivable (like a pure sine wave tone).
From what I understand of the marketing, the part of the signal that an MP3 encoder strips out is exactly where they would place their data. They can stick any data they want in there and we just plain won't hear it, but a computer+microphone doesn't have this problem.
Interesting corrolary: The time frame this will work in is limited, as digital transmission usually uses compressed audio, and the act of compressing the audio will preferentially eliminate this data. (Or does digital radio transmit an uncompressed stream?) They'd better get marketing this now, so that there's an installed base and they can try to later create receivers that will re-add their signal on the receiving side. Of course, if all anybody is using this for is advertising, I can't imagine we're going to go out of our way to buy "Advertising Enabled!" digital radio receivers.
Re:RDS has been around for ages (Score:3, Informative)
RDS signals are not only inaudible they are heavily filtered out before the audio stages of the receiver.
A european FM radio carrier has mono audio
at 0-15KHz, a pilot tone for the stereo decoder at
19KHz, a stereo difference signal around 20-35KHz
and RDS data above that.
Compare it with RDS (Score:4, Informative)
- Show the Station name in your radio display
- Show what's playing
- Certain stations are transmitted over several frequencies. RDS knows the alternative frequencies of your stations and automatically switches to the best frequency
Re:Color encoded in black-and-white signal! (Score:3, Informative)
Orginally, the NTSC chose CBS Color ( a mechanically-timed color wheel system ) because RCA's "no moving parts" system was late and looked terrible.
But RCA had an established Black and White user base, and the CBS color sets were incompatible, so CBS color sets didn't sell. A few years later, NTSC formally retracted their endorsement of CBS Color and endorsed RCA color, which hasn't changed for 48 years.
I only wonder if the same shiznit is going to happen to HDTV, we'll be stuck in 480i land forever!
Re:bandwidth? (Score:3, Informative)
Let's assume some rockin' speakers with a 22kHz rolloff, and a great FM reception with 96dB Signal-to-Noise (and a very quiet listening room). That's approximately 16 bits at 44kHz or 704Kb/s of information. That has to carry both the audio and the data signal. The data signal would have to be mono, since most toys & cell phones don't have two ears (err, microphones). Now, you have a 704Kb/s bandwidth in which you only need about 1/5 = 140Kb/sec for good audio, leaving you with a theoretical maximum of 563Kb/s left for data. Put in some forward error correction and packet and coding and other overhead and you'll probably get something more akin to 200Kb/s.
But wait! Let's assume a car with some poor tweeters with a 15kHz rolloff, and poor FM reception with 65dB Signal-to-Noise with road noise added in. That's approximately 11 bits at 7.5kHz or 82Kb/s of total information. Ooops! You've exceeded your channel capacity by almost 2x, and you'll pretty much get a big fat zero data bits.
So, the makers have make a tradeoff ->
1) Low data rates: significantly less than ~200kB/sec to accomodate cheap stereos but retain audio quality.
2) Poor audio quality: significantly less than ~140kB/sec to accomodate higher data rates or cheap stereos.
3) Lose functionality on cheap stereos: but retain both good data rates and quality audio for those who can receive it.
My guess is that they'll just go to something tiny like 500b/s, in order to reach the most market share. Even at that rate, a text ad would come through right quick.
I can just see the next Furby craze, now they get instructions (programming??) from the TV!
Anyone know the max bandwith and SNR of NTSC audio?
Subaudible tones already used (Score:4, Informative)
Of course, then there is IBOC from Ibiquity [ibiquity.com] which is an on-channel digital enhacement for AM and FM signals, part of which could be used for datacasting [beradio.com], as part of most DTV signals will.
Re:Why not a perfect cutoff? (Score:4, Informative)
At the recorder, you must cutoff signals over 22050 or risk the horrible problem of aliasing (again, out of scope of a Slashdot post but pretty interesting). Since you can't have a perfect cutoff filter, you generally can't record 22049Hz signals except with extreme attenuation (in the specific case of 22049, it will well below the noise floor). Generally, when the CD players re-construct the sound, they will also do some filtering as a side-effect of how they do it. So you can't generally play back a 22049Hz signal either, even if you directly encode it onto a CD.
So while you can encode it, you can't record it directly and you can't play it back, so in a very real practical sense, 22049Hz is not usable with CDs. And so on and so forth for the other frequencies between 20000 and 22050. It's a smooth curve (and not necessarily the same one for two pieces of equipment, though my impression is that they have standardized somewhat because it's cheaper that way), so in a real-world CD recording and playback application, in a very real way there's no particular cutoff frequency you can directly point at, even though there's one in theory.
In general, it's a pretty pedantic point.
This, by the way, is part of the reason that CD's sample at 44100, instead of 40000. 40000 would be somewhat more efficient with the storage medium, but you'd have problems with the fact that you have no room to filter out the higher frequencies without hitting "good" ones as well. There are other concerns too, that's not the whole story, but it is a very significant part of it. In fact that goes for this whole post; I'm skimming over a lot because this is only a Slashdot post. (Like "20-20,000 is only a convenient fiction", the exact way filters behave, etc.)