Remove the resampler. #95
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| mink = st->samperr - 10; | ||
| maxk = st->samperr + 10; | ||
| mink = FFTCP / 2 - 25; | ||
| maxk = FFTCP / 2 + 25; |
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I had to widen the search range here to account for ppm error. Each invocation of acquire_process processes 64 * 2160 samples, so 100 ppm error causes a 14-sample offset between invocations.
| float phase, slope; | ||
| float complex sum; | ||
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| sum = 0; | ||
| for (int r = 0; r < BLKSZ; r++) | ||
| sum += buf[ref * BLKSZ + r] * buf[ref * BLKSZ + r]; | ||
| phase = cargf(sum) * 0.5; |
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This method of calculating the phase only works when the slope is very small. Error in the sampling frequency increases the slope, so I had to change it to take the slope into account.
| sum = 0; | ||
| for (int r = 0; r < BLKSZ; r++) | ||
| sum += buf[ref * BLKSZ + r] * buf[ref * BLKSZ + r] * cexpf(-I * 2 * slope * r); | ||
| phase = cargf(sum) * 0.5; |
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The phase calculation now takes place after the slope is calculated, so that the slope can be subtracted out.
I merged calc_phase into adjust_ref because this method needs to use the averaged slope. (I tried with the raw slope, but that reduced the receiver's weak-signal performance.)
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The changes to acquire.c are simpler to review with whitespace changes ignored: https://github.com/theori-io/nrsc5/pull/95/files?w=1 |
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@classicjazz @pclov3r You may be interested in testing this out. It should reduce the acquisition time by about 3 seconds. @mrbubble62 I expect this change will improve ARM performance significantly. |
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@argilo This is a great change! When the gain is set or if you set it manually around 6 seconds until actual audio playback. |
Currently nrsc5 uses an arbitrary resampler to maintain time synchronization. This approach has some significant drawbacks:
resamp_q15_execute.acquire_processspends more than three seconds estimating the resampling rate before it starts passing samples through tosync_push.The resampler can be eliminated simply by modifying
acquire_processto skip or re-use samples as necessary to maintain synchronization. It already carries over one symbol's worth of samples from one invocation to the next, so I modified it to carry over anywhere from 0.5 to 1.5 symbols depending on the value ofsamperr. It chooses the number of samples so as to target asamperrvalue ofFFTCP / 2on the following invocation.A couple other small changes were necessary to make this work with RTL-SDRs that have ±100 ppm crystals. I'll note those in the code.
I tested this by changing the sample rate in main.c to
1488375 - 200and1488375 + 200(thus simulating an error of 134 ppm) and using a couple old non-TCXO RTL-SDR's I had lying around. I also processed about 15 recordings I had made of stations at various signal strengths, and found that the reported MER values were comparable before & after the change.