Browse Prior Art Database

MATCHED AUDIO QUALITY FILTERING TECHNIQUE

IP.com Disclosure Number: IPCOM000007822D
Original Publication Date: 1996-Aug-01
Included in the Prior Art Database: 2002-Apr-25
Document File: 3 page(s) / 178K

Publishing Venue

Motorola

Related People

Tom McClaughry: AUTHOR [+2]

Abstract

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.

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Technical Developments

MATCHED AUDIO QUALITY FILTERING TECHNIQUE

by Tom McClaughry and Bob LoGalbo

BACKGROUND

is transmitting and the brand of the receiving unit. In cases where both the transmit and receive paths use some level of spectral shaping, the effects will add and produce some appreciable amount of vari- ability in the presented audio. This condition will be undesirable to the user as the "sound" of his audio may vary considerably depending on which APCO-2S compliant radio is talking to him. This issue already exists in analog radio systems today when they are inter-operated with other vendor's equipment, and has been noted as a customer com- plaint. The emergence of an APCO-25 standard- driven market will further aggravate this problem.

  Additionally, use ofradio accessories (e.g. public safety lapel microphone/speaker, surveillance microphone/speaker, etc.) on the subscriber units have been seen to add a large amount of variability to the delivered audio to the user. This is also unde- sirable to the user listening to the audio as the qual- ity ofthe voice will vary with the presence or type of accessory, Similarly, Mobile and Portable units often vary considerably in their audio response.

  Consequently, in order to Molly satisfy customer expectations of delivered audio quality, a method is required to assure some level of consistency to the audio chracteristics presented to the radio user.

  The establishment of the Association of Public Safety and Communications Officers Project 25 (APCO-25) Common Air Interface (CAI) interim standard has created a private radio user environ- ment whereby radios developed and manufactured from multiple vendors are expected to inter-operate with acceptable and consistent levels of voice qual- ity. The APCO-25 CA1 documentation requires all manufacturer's of compliant equipment to use a par- ticular voice coder (Improved Multi-band Excita- tion), as well as pass a conformance test that will require a minimum Mean Opinion Score (MOS).

PROBLEM

  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 "brig...