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us144mkii_playback: clamp feedback PLL to prevent DAC underflow noise 

Ignore transient bad device feedback that would drive freq_q16 to zero or
an unrealistic value, and clamp freq_q16 within 75%%-125%% of nominal.
Also force a minimal nominal packet if a zero-length playback URB is
produced, preventing the device DAC from underflowing and outputting
persistent noise.
This commit is contained in:
Marvin 2026-06-25 10:37:23 -03:00
parent 32986d7934
commit db1de1f9c1
1 changed files with 32 additions and 3 deletions
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29
us144mkii_playback.c
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@ -256,6 +256,22 @@ void playback_urb_complete(struct urb *urb)
}
urb->transfer_buffer_length = total_bytes;
/* A stuck PLL can produce all-zero-length packets, which makes the
* device underflow and output persistent noise. Force a minimal
* nominal packet if that happens.
*/
if (total_bytes == 0) {
tascam->freq_q16 = div_u64(((u64)tascam->current_rate << 16), 8000);
tascam->phase_accum = 0;
frames = min((u32)(tascam->current_rate / 8000), (u32)MAX_FRAMES_PER_PACKET);
if (frames == 0)
frames = 1;
urb->iso_frame_desc[0].offset = 0;
urb->iso_frame_desc[0].length = frames * PLAYBACK_FRAME_SIZE;
total_bytes = urb->iso_frame_desc[0].length;
urb->transfer_buffer_length = total_bytes;
}
/* Ghost Playback: Send Silence */
if (!atomic_read(&tascam->playback_active)) {
if (tascam->running_ghost_playback) {
@ -342,12 +358,25 @@ void feedback_urb_complete(struct urb *urb)
u8 *d = urb->transfer_buffer + urb->iso_frame_desc[p].offset;
u32 val = (urb->iso_frame_desc[p].actual_length >= 3) ? (d[0] + d[1] + d[2]) : (d[0] * 3);
u32 target = (val << 16) / 24;
u32 nominal = div_u64(((u64)tascam->current_rate << 16), 8000);
u32 min_freq = nominal - (nominal >> 2);
u32 max_freq = nominal + (nominal >> 2);
/* Ignore transient bad feedback that would drive the PLL to
* zero or an unrealistic value and cause DAC underflow noise.
*/
if (tascam->current_rate > 0 && target >= min_freq && target <= max_freq) {
tascam->freq_q16 = (tascam->freq_q16 * PLL_FILTER_OLD_WEIGHT +
target * PLL_FILTER_NEW_WEIGHT) / PLL_FILTER_DIVISOR;
if (tascam->freq_q16 < min_freq)
tascam->freq_q16 = min_freq;
else if (tascam->freq_q16 > max_freq)
tascam->freq_q16 = max_freq;
tascam->feedback_synced = true;
}
}
}
}
spin_unlock_irqrestore(&tascam->lock, flags);
usb_anchor_urb(urb, &tascam->feedback_anchor);