MATCHED AUDIO QUALITY FILTERING TECHNIQUE
Original Publication Date: 1996-Aug-01
Included in the Prior Art Database: 2002-Apr-25
While the MOS Conformance test will assure that all manufacturer's compliant radios meet the minimum levels of intelligibility when inter- operating, no specification exists for audio shaping in the radio to meet a customer's particular audio preferences. For example, one compliant vendor may utilize a particular amount of low-end audio empha- sis in the lower portion of the audio passband for its receiver while another compliant vendor might use a high-end audio emphasis. Both of these conditions do not effect the ability of the radio to meet the APCO-25 Vocoder Conformance Test, yet each will deliver its own "unique" quality audio to the user. For radios that do not inter-operate with other ven- dors, this does not present a particular problem. A vendor is merely noted as having a particular "sound" to their radios. For example, Motorola mobile radios have traditionally used some amount of high-end emphasis in their microphones, which creates a "brighter" quality to the audio. Customers have come to expect this audio characteristic and can perceive a flatter response as being "muted" or "dull? How- ever, in the situation where compliant radios from multiple vendors are inter-operating, the audio char- acteristics presented to the user by the receiving unit will vary considerably with the brand of radio that SOLUTION Refer to Figure 1. The technique described here resolves the problem stated above in the following way. The transmitting unit is designed such that the spectral response of the transmit audio path is stored in the unit's electronic codeplug or personality bank. This information is referred to as the "Transmit Spec- tral Response Tag" and could be represented in such a way as to use a small amount of storage making the tag efficient of bandwidth when it is transmitted across the RF channel. (For example, a 16 bit rep- resentation could be used where the 300 to 3000 Hertz audio passband is represented as 8 distinct D Moromla. Inc. ,996 146 August1996 M-LA Technical Developments frequency bands with 4 possible amplitude levels per band. Figure 1 shows a transmit response that has a high-end boost at -2500 Hz represented as a 16 bit number). The spectral response information used to create the tag could have been acquired l?om audio analysis of that model of radio platform. If a trans- mit accessory is in use, the radio could be designed to detect the presence and type of accessory (through some fixed identifier presented to the radio in the electrical connection to the accessory) or, more sim- ply, account for the use of the accessory when the tag is loaded into the radio codeplug or personality bank. In either case, the Transmit Spectral Response Tag would incorporate the audio passband response of whatever circuitry/software existed between (and including) the microphone and the Analog-to-Digital converter stage. The Tag information could also be adaptable by the user through soft-keys, or by a serv- ice technician through the Radio Service Software. This would allow for flexibility in accessory usage.