pa_usec_t sink_latency;
pa_usec_t last_rate_update;
+ pa_usec_t last_latency;
};
struct userdata {
pa_atomic_store(&s->timestamp, (int) now.tv_sec);
if (s->last_rate_update + RATE_UPDATE_INTERVAL < pa_timeval_load(&now)) {
- pa_usec_t wi, ri, render_delay, sink_delay = 0, latency, fix;
- unsigned fix_samples;
+ pa_usec_t wi, ri, render_delay, sink_delay = 0, latency;
uint32_t base_rate = s->sink_input->sink->sample_spec.rate;
uint32_t current_rate = s->sink_input->sample_spec.rate;
uint32_t new_rate;
+ double estimated_rate;
pa_log_debug("Updating sample rate");
pa_log_debug("Write index deviates by %0.2f ms, expected %0.2f ms", (double) latency/PA_USEC_PER_MSEC, (double) s->intended_latency/PA_USEC_PER_MSEC);
- /* Calculate deviation */
- if (latency < s->intended_latency)
- fix = s->intended_latency - latency;
- else
- fix = latency - s->intended_latency;
-
- /* How many samples is this per second? */
- fix_samples = (unsigned) (fix * (pa_usec_t) s->sink_input->thread_info.sample_spec.rate / (pa_usec_t) RATE_UPDATE_INTERVAL);
-
- if (latency < s->intended_latency)
- new_rate = current_rate - fix_samples;
- else
- new_rate = current_rate + fix_samples;
+ /* The buffer is filling with some unknown rate R̂ samples/second. If the rate of reading in
+ * the last T seconds was Rⁿ, then the increase in buffer latency ΔLⁿ = Lⁿ - Lⁿ⁻ⁱ in that
+ * same period is ΔLⁿ = (TR̂ - TRⁿ) / R̂, giving the estimated target rate
+ * T
+ * R̂ = ─────────────── Rⁿ . (1)
+ * T - (Lⁿ - Lⁿ⁻ⁱ)
+ *
+ * Setting the sample rate to R̂ results in the latency being constant (if the estimate of R̂
+ * is correct). But there is also the requirement to keep the buffer at a predefined target
+ * latency L̂. So instead of setting Rⁿ⁺ⁱ to R̂ immediately, the strategy will be to reduce R
+ * from Rⁿ⁺ⁱ to R̂ in a steps of T seconds, where Rⁿ⁺ⁱ is chosen such that in the total time
+ * aT the latency is reduced from Lⁿ to L̂. This strategy translates to the requirements
+ * ₐ R̂ - Rⁿ⁺ʲ a-j+1 j-1
+ * Σ T ────────── = L̂ - Lⁿ with Rⁿ⁺ʲ = ───── Rⁿ⁺ⁱ + ───── R̂ .
+ * ʲ⁼ⁱ R̂ a a
+ * Solving for Rⁿ⁺ⁱ gives
+ * T - ²∕ₐ₊₁(L̂ - Lⁿ)
+ * Rⁿ⁺ⁱ = ───────────────── R̂ . (2)
+ * T
+ * Together Equations (1) and (2) specify the algorithm used below, where a = 7 is used.
+ */
+ estimated_rate = (double) current_rate * (double) RATE_UPDATE_INTERVAL / (double) (RATE_UPDATE_INTERVAL + s->last_latency - latency);
+ pa_log_debug("Estimated target rate: %.0f Hz", estimated_rate);
+ new_rate = (uint32_t) ((double) (RATE_UPDATE_INTERVAL + latency/4 - s->intended_latency/4) / (double) RATE_UPDATE_INTERVAL * estimated_rate);
+ s->last_latency = latency;
if (new_rate < (uint32_t) (base_rate*0.8) || new_rate > (uint32_t) (base_rate*1.25)) {
pa_log_warn("Sample rates too different, not adjusting (%u vs. %u).", base_rate, new_rate);
pa_timeval_load(&now),
TRUE);
s->last_rate_update = pa_timeval_load(&now);
+ s->last_latency = LATENCY_USEC;
pa_atomic_store(&s->timestamp, (int) now.tv_sec);
if ((fd = mcast_socket((const struct sockaddr*) &sdp_info->sa, sdp_info->salen)) < 0)