We claim:
1. An Airborne Mark 4 SELCAL Decoder & Implementation method there of comprising of;
- An analog signal conditioning (2) circuitto match the input audio impedance followed by an audio codec (3) for sampling the audio data at 16kHz.
- An AGC scheme (4) where the target audio level is maintained at constant -5.5dB irrespective of input signal variation. This followed by a band pass filtering (5) network to remove the unwanted signal outside of 300-1600Hz.
- A code detection process (6) where in the computed tone magnitude (6A) is compared against tone pair threshold (6B) followed by time duration computation (6C) to give final code validation (6D).
2. The analog signal conditioning (2) circuitas claimed in claim 1 is built on with balanced audio line filters, audio transformer & low noise audio amplifier to smoothen the input signal from any spike & interferences.
3. The AGC scheme (4) as claimed in claim 1 is built on setting the audio target level to -5.5&B with maximum PGA gain of 31.5dB. Also the noise gate threshold is set to -40dB such that any interfering signal/noise below l.OmV wiH be blocked
4. The 300-1600Hz band pass filtering (5) as claimed in claim 1 is built on passing the audio signal through a 71st order Low pass filter followed by 148th order High Pass filter. Minimum attenuation of 60dB in the stop band is obtained.
5. The tone magnitude computation (6A) as claimed in claim 1 is built on dynamic configuration of four 2 nd Order IIR filters based on assigned SELCAL code& filter magnitude calculation considering block processing of 2048 samples at a time.
6. The tone pair threshold assessment (6B) as claimed in claim 1 built on:
a. Threshold computation (6B1) of individual tones based on its centre frequency, probable interfering frequencies including harmonics & noise, in band 0.15% frequencies and out band 1.65% frequencies.
b. Various scale factors like harmonic distortion factor (6B2) of 90%, dual tone scale factor (6B3) of 60% 8s tone imbalance factor (6B4) of 25% applied on the tone threshold values as per sensed signal condition.
7. The time duration computation (6C) as claimed in claim 1 built on:
a. Detection of tone pair start point (6C1) & tone pair stop point (6.C2). This obtained through capturing the time stamps coordinates in terms of block number & sample number for these events. One block data collection takes 128mSec 8s one sample at every 62.5uSec.
b. Using the captured time stamps the pulse & pause duration computation (6C3) takes place which is the subtraction of time stamps followed by multiplication of applicable block 8& sample durations.
8. The code validation (6D) as claimed in claim 1 built on checking of aH previous validity flags, tone duration comparison against set limits 8s check on legal code rules. This ensures the final validity what goes to the lamp 85 chime activation (7) circuit to alert the crew accurately.