Tandem signaling
Encyclopedia
Tandem signaling is one of the most difficult conditions for speech coders to perform well in is the case where a digital speech-coded signal is transmitted from the mobile to the base station, and then demodulated into an analog signal which is then speech coded for retransmission as a digital signal
over a land line or wireless link. This situation, called tandem signaling, tends to exaggerate the bit errors originally received at the base station. Tandem signaling is difficult to protect against, but is an important evaluation criterion in the evaluation of speech coders. As wireless systems proliferate, there will be a greater demand for mobile-to-mobile communications, and such links will, by definition, involve at least two independent, noisy tandems.
If two users are on two different mobile phone networks, with different audio codecs (such as AMR-Narrowband and EVRC) falling back to the lowest possible birate on each end (4.75, ~6 kbit/s), then the speech will suffer from problems. Tandeming is nearly impossible to avoid, because even if all mobile phones used one audio codec, there is the issue of packet retransmission and re-encoding. When a user makes a call to a POTS
phone, the packets "flow" to the tower in that particular cell, and are then decoded into 64 kbit/s PCM. This creates little to no extra noise, and if the signal is sufficient enough, the user on the POTS line will be able to carry on a conversation. If, however, the user on the mobile phone calls another mobile phone, the packets cannot simply be 'shifted' to the other user's mobile phone. Instead, they must be sent to the tower, decoded into PCM, sent to the other corresponding tower, and then transcoded to the audio codec used on the other handset. This results in a possible loss of speech quality.
Tandeming can be demonstrated using a personal computer, a microphone, and Apple's QuickTime Pro software, version 6.5 or above:
Using this simple tandem experiment, you can actually hear what goes on in the real-world for mobile phone users day to day.
To the human ear, there will be a discernible loss of audio quality. (It is to be noted, however, that the degree to which any given human ear will be able to determine said loss of audio quality is subjective and varies substantially.)
Digital signal
A digital signal is a physical signal that is a representation of a sequence of discrete values , for example of an arbitrary bit stream, or of a digitized analog signal...
over a land line or wireless link. This situation, called tandem signaling, tends to exaggerate the bit errors originally received at the base station. Tandem signaling is difficult to protect against, but is an important evaluation criterion in the evaluation of speech coders. As wireless systems proliferate, there will be a greater demand for mobile-to-mobile communications, and such links will, by definition, involve at least two independent, noisy tandems.
Tandeming examples and issues
If two individuals are using mobile phones on two different networks, and one attempts to call the other, tandeming will occur. This is even more evident, in terms of speech intelligibility, when one user is using a phone with a different audio codec than the other user. If one user has a high-bitrate audio codec, and the other uses has a low-bitrate, the user with the high-bitrate will be able to hear the user with the low-bitrate, as the tandeming will not have enough of an opportunity to make the speech completely unintelligible.If two users are on two different mobile phone networks, with different audio codecs (such as AMR-Narrowband and EVRC) falling back to the lowest possible birate on each end (4.75, ~6 kbit/s), then the speech will suffer from problems. Tandeming is nearly impossible to avoid, because even if all mobile phones used one audio codec, there is the issue of packet retransmission and re-encoding. When a user makes a call to a POTS
Plain old telephone service
Plain old telephone service is the voice-grade telephone service that remains the basic form of residential and small business service connection to the telephone network in many parts of the world....
phone, the packets "flow" to the tower in that particular cell, and are then decoded into 64 kbit/s PCM. This creates little to no extra noise, and if the signal is sufficient enough, the user on the POTS line will be able to carry on a conversation. If, however, the user on the mobile phone calls another mobile phone, the packets cannot simply be 'shifted' to the other user's mobile phone. Instead, they must be sent to the tower, decoded into PCM, sent to the other corresponding tower, and then transcoded to the audio codec used on the other handset. This results in a possible loss of speech quality.
Tandeming can be demonstrated using a personal computer, a microphone, and Apple's QuickTime Pro software, version 6.5 or above:
- Record a fictional conversation with your microphone, ensuring that the audio levels are sufficient but not too high.
- Save the file was a WAVE, AIFF, or any uncompressed format.
- Open the file in the QuickTime player.
- Choose to export to QuickTime movie, and choose AMR-Narrowband at 12.2 kbit/s, 8 kHz, mono.
- Give the file a name; export the file.
- Open up the exported file, and export another file. This time, change the audio codec to Qualcomm Pure voice, 8 kHz, mono, half-rate, optimize for streaming.
- Give the file a new name, then save it.
- Compare the two files (aurally).
Using this simple tandem experiment, you can actually hear what goes on in the real-world for mobile phone users day to day.
To the human ear, there will be a discernible loss of audio quality. (It is to be noted, however, that the degree to which any given human ear will be able to determine said loss of audio quality is subjective and varies substantially.)