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3.2.2 Signal to noise ratio shall be better than 40 dBA (38dB) when the reference signal is replayed (1072)
3.2.3 Harmonic distortion of the reference signal, replayed at 0dBm, shall not exceed 2.5% (1073)
3.2.4 Crosstalk from adjacent channels shall not exceed 40 dB (1074)
3.2.5 Wow and Flutter shall not exceed 1% (1075)
4 Digital Equipment
Digital recording equipment can be classified as that which encodes and records analogue voice or data communications onto internal storage with the capability to transfer the recorded data into archive storage periodically to achieve the minimum retention criteria or on demand for investigative purposes. Magnetic/optical media or storage drives utilising electronic, electrical or mechanical devices may be used in this process.
4.1 Analogue to Digital Conversion
4.1.1 Voice Coding and Decoding (Codec) Techniques
4.1.1.1 The recording equipment or system shall employ voice coding techniques which ensure the replay quality of previously archived radio communication messages will achieve a minimum Mean Opinion Score (MOS) of 4.0 (Good) (2268).
4.1.1.2 The voice coding scheme shall be able to cope with different types of voice, multiple voices, background noise without any significant deterioration in quality (1077).
4.1.1.3 The voice coding scheme should comply with published European or International standards where available (1078).
NOTE: Voice coding schemes using waveform coding techniques include CCITT
G.711 - A/μ-law PCM and CCITT G.726-ADPCM. Codecs using these techniques have been scored in accordance with ITU (T) Recommendation
P.862.1. The results obtained from these tests have indicated that the use of codecs which employ either G.711 (at 64 kbit/second) or G.726 (at 32 kbit/ second) will meet or exceed the minimum MOS required for the replay of radio communications between ATC and aircraft under their control.
4.1.2 Data Compression
4.1.2.1 The amount of data compression applied at the analogue to digital conversion, either as part of the voice coding scheme or as a separate process, shall not significantly degrade the recorded communications (1079).
NOTE: Based upon trials involving the subjective assessment of data compression of speech for Air Traffic Control Applications, a minimum MOS of 3.5 is acceptable for the recording and subsequent replay of telephone communications.
4.2 Intermediate Storage
4.2.1 Where an internal storage device is used, the process by which the communications are routinely transferred onto a separate and secure archive storage system, where necessary to meet minimum retention criteria, shall be automatic (not requiring human intervention) (1080) and shall be secure from attempts to select, alter or interfere in any way with the data (1081).
4.2.2 The information held on the intermediate storage devices shall be transferred to the archive storage system via an appropriate communication system at regular intervals (2269).
4.2.3 The equipment shall use a safe shutdown mode, in the event of power failure or equipment malfunction, to ensure that intermediate storage data is not lost and that the communications can be replayed normally from the archive storage system (1083).
4.3 Archive Storage System
4.3.1 The equipment shall use a safe shutdown mode, in the event of power failure or equipment malfunction, to ensure that any necessary file management information can be written to the archive storage system, so that the communications can be replayed normally from the archive storage system (1084).
4.3.2 Recording Check
4.3.2.1 Devices and/or techniques shall be incorporated to provide a check for successful recording onto the archive storage system (1085).
NOTE: Read after write verification between the intermediate and archive storage systems can be used.
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CAP 670 Air Traffic Services Safety Requirements 2(5)
