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AES67 TX: send each packet right after its last sample is due

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The periodic TX timer had an arbitrary phase against packet boundaries,
so packets waited up to one packet time (different after every restart).
- One-shot esp_timer aimed at the next packet's due time from PTP, with
  the clock re-read after sending.
- 1 kHz tone from a per-rate lookup table (one period = rate/1000
  samples) instead of sinf() per sample.
Measured (RTP time - arrival, 4 ms vs 1 ms packets, expected -3.07 ms
incl. wire time): -3.3 ms extra before, now -3.14 ms. Tone within
1.0 LSB of the ideal PTP-phased sine, no gaps.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
This commit is contained in:
Ben Nicholson
2026-09-25 06:48:47 +10:00
parent bae6a8a2f9
commit 044e9913a7
1 changed files with 21 additions and 7 deletions
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components/aes67_tx/aes67_tx.c
+21 -7
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@@ -22,6 +22,7 @@
#define MAX_LAG_NS 20000000 // more than 20 ms behind: resync instead of bursting
#define TONE_HZ 1000
#define TONE_DBFS -18.0
#define WAKE_MARGIN_US 20 // wake just after a packet's last sample is due
static const char *TAG = "aes67_tx";
@@ -166,15 +167,23 @@ static int64_t samples_to_ns(int64_t s, int rate)
}
// 1 kHz tone, phase from the PTP sample index, so every sender's tone lines up.
// One period is rate / 1000 samples (48 at 48 kHz, 96 at 96 kHz): precomputed table.
static void tone(int32_t *buf, int frames, int channels, int rate, int64_t s0)
{
static float amp;
if (!amp) {
amp = (float)(pow(10.0, TONE_DBFS / 20.0) * 2147483647.0);
}
static int32_t table[96000 / TONE_HZ];
static int table_rate;
int period = rate / TONE_HZ;
if (table_rate != rate) {
double amp = pow(10.0, TONE_DBFS / 20.0) * 2147483647.0;
for (int i = 0; i < period; i++) {
table[i] = (int32_t)(amp * sin(2.0 * M_PI * i / period));
}
table_rate = rate;
}
int ph = (int)(s0 % period);
for (int i = 0; i < frames; i++) {
int32_t v = (int32_t)(amp * sinf(2.0f * (float)M_PI * (float)((s0 + i) % period) / (float)period));
int32_t v = table[ph];
ph = ph + 1 == period ? 0 : ph + 1;
for (int ch = 0; ch < channels; ch++) {
buf[i * channels + ch] = v;
}
@@ -216,8 +225,6 @@ static void tx_task(void *arg)
next = 0;
if (c.enabled) {
fd = open_socket(&c);
// wake once per packet time; late wakes are caught up below
esp_timer_start_periodic(timer, (uint64_t)c.frames * 1000000ULL / c.rate);
}
}
@@ -277,6 +284,13 @@ static void tx_task(void *arg)
seq++;
next += c.frames;
}
// Wake when the next packet's last sample is due (PTP time), not on a free-running
// period, so each packet leaves right after it is complete. Re-read the clock:
// building and sending took time.
aes67_ptp_now_ns(&now);
int64_t wait_us = (samples_to_ns(next + c.frames, c.rate) - now) / 1000 + WAKE_MARGIN_US;
esp_timer_stop(timer);
esp_timer_start_once(timer, wait_us < 50 ? 50 : wait_us > 10000 ? 10000 : wait_us);
}
if (new_state != state) {
state = new_state;