CBET
 | |
|---|---|
| Frequencies | 88 MHz,108 MHz,520 Hz,1.7 MHz |
| Frequency Range | 520 Hz - 108 MHz |
| Mode | — |
| Modulation | FM,AM |
| ACF | — |
| Emission Designator | — |
| Bandwidth | 2 kHz |
| Location | |
| Short Description | CBET "Critical Band Encoded Tones" are tones added into radio broadcast's audio output for identification purposes. |
| I/Q Raw Recording | — |
| Audio Sample | |
CBET/eCBET tones are a set of 10 frequency modulated tones between 1-3kHz. These signals are inserted into broadcast audio in over 50 major markets for identification by "Portable People Meters" used in Nielsen's rating scheme. Nielsen controversially purports that these signals are encoded in such a way that they are 'masked' by the program audio.
There are 10 bands, each of which can carry 1 of 18 tones. There are 16 tones to a signal, so the stream is 4 bits wide, and the other two tones are used as "STOP" and "SYNC" markers. This allows for an data rate of 8 bits per second to be encoded onto the audio. All bands carry identical information, but each bit within a band is encoded with a different tone from any of the other bands. This appears random or uncorrelated at first glance, but the pattern emerges pretty quickly if you do a long enough analysis. The same message gets identically repeated 12 to 13 times per minute and the message only changes once per minute.
Samples[edit]
Recording from WQXR:
Frequencies[edit]
Broadcast FMFrequency Modulation/AMAmplitude Modulation
Decoding Software[edit]
Proprietary modulation scheme.
