Circuit Merit
From Wikipedia, the free encyclopedia
The Circuit Merit system is a measurement process designed to assess the voice-to-noise ratio in wired and wireless telephone circuits,[1] especially the AMPS system,[2] and although its reporting scale is sometimes used as input for calculating mean opinion score,[3] the rating system is officially defined relative to given ranges of voice-to-noise ratios.
Various technical sources state that experimental research vary in what ratio is required for good understanding, but is typically above 20 dB, and noticeably higher reports of voice quality can be achieved when the ratio is near 30 dB.
There are 5 levels of quality, detailed as follows:
| Circuit Merit | Label[2][4] | Definition | Nominal Value (speech-to-noise ratio) | Range |
|---|---|---|---|---|
| CM5 | Excellent | Perfectly readable, negligible noise. | -- | >30 dB |
| CM4 | Good | Perfectly readable but with noticeable noise. | 22 dB | 16 to 30 dB |
| CM3 | Fair | Readable with only occasional repetition (commercial). | 12 dB | 9 to 16 dB |
| CM2 | Poor | Readable with difficulty, requires frequent repetition (noncommercial). | 7 dB | 5 to 9 dB |
| CM1 | Unsatisfactory | Unusable, presence of speech barely discernible. | -- | <5 dB |
Measurement of audio speech-to-noise ratios in a way which reflects interfering effects in a meaningful manner is difficult for the kinds of noise often encountered at mobile system receivers. For this reason and as a matter of convenience, a subjective rating of the interfering effect of the noise using the term “circuit merit” is commonly used in place of metered measurements. This method uses a scale of five steps to describe performance. These are listed and defined in the above table. The speech-to-noise ratios in dB included in this table are arbitrary numbers which apply if both speech and noise are measured on either a 2B noise measuring set with F1A line weighting or a 3A noise measuring set with C-message weighting. In making such measurements, noise is measured in the normal manner and speech volume is read by the method used with a VU meter.
Circuit Merit 3 is generally regarded as the minimum acceptable for public mobile telephone service, and is used as a basis for drawing coverage boundaries. Where necessary, CM2 may be found tolerable for occasional calls, but this grade of transmission is clearly unsuitable as a service offering. In typical service areas so defined, users will experience transmission as poor as CM3 only when the mobile station is situated near the fringes of the area. They will enjoy better transmission throughout the bulk of the coverage area and, as a usual consequence, on most of their calls. Situations in which fringe coverage areas pass through important localities heavily frequented by local mobile stations should be avoided since users may not find CM3 agreeable if experienced on any large proportion of their calls.
For use on digital voice telephone systems, the DAQ reference is used. This differs from Circuit Merit in that it adds definitions for digitized voice and a static SINAD for multipath fading, using levels described in TIA/EIA TSB-88-A.[5]
| Delivered Audio Quality | Definition |
|---|---|
| DAQ 5 | Speech easily understood. |
| DAQ 4.5 | Speech easily understood. Infrequent Noise/Distortion. |
| DAQ 4 | Speech easily understood. Occasional Noise/Distortion. |
| DAQ 3.4 | Speech understandable with repetition only rarely required. Some Noise/Distortion. |
| DAQ 3 | Speech understandable with slight effort. Occasional repetition due to Noise/Distortion. |
| DAQ 2 | Understandable with considerable effort. Frequent repetition due to Noise/Distortion. |
| DAQ 1 | Unusable. Speech present but not understandable. |
