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Clean up and bug-fix the code for estimating remote-to-local clock drift using least squares linear regression. Use the estimated remote-to-local difference and the calculated drift instead of just using the difference estimated from the last-exchanged timing message.

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Mike Brady
2020-06-09 11:37:24 +01:00
parent 1312772bc8
commit 9f9b36c01f
1 changed files with 27 additions and 58 deletions
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rtp.c
+27 -58
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@@ -602,17 +602,13 @@ void *rtp_timing_receiver(void *arg) {
// debug(1,"Return trip time: %" PRIu64 " nS, remote processing time: %" PRIu64 "
// nS.",return_time, remote_processing_time);
uint64_t local_time_by_remote_clock = distant_transmit_time + return_time / 2;
// remove the remote processing time from the record of the return time, as long at the
// processing time looks sensible.
if (remote_processing_time < return_time)
return_time -= remote_processing_time;
else
debug(1, "Remote processing time greater than return time -- ignored.");
int cc;
// debug(1, "time ping history is %d entries.", time_ping_history);
for (cc = time_ping_history - 1; cc > 0; cc--) {
conn->time_pings[cc] = conn->time_pings[cc - 1];
// if ((conn->time_ping_count) && (conn->time_ping_count < 10))
@@ -625,12 +621,9 @@ void *rtp_timing_receiver(void *arg) {
}
// these are used for doing a least squares calculation to get the drift
conn->time_pings[0].local_time = arrival_time;
conn->time_pings[0].remote_time = distant_transmit_time;
conn->time_pings[0].remote_time = distant_transmit_time + return_time / 2;
conn->time_pings[0].sequence_number = sequence_number++;
conn->time_pings[0].chosen = 0;
conn->time_pings[0].local_to_remote_difference =
local_time_by_remote_clock - arrival_time;
conn->time_pings[0].dispersion = return_time;
if (conn->time_ping_count < time_ping_history)
conn->time_ping_count++;
@@ -653,14 +646,14 @@ void *rtp_timing_receiver(void *arg) {
// uint64_t local_time_chosen = arrival_time;
// uint64_t remote_time_chosen = distant_transmit_time;
// now pick the timestamp with the lowest dispersion
uint64_t l2rtd = conn->time_pings[0].local_to_remote_difference;
uint64_t rt = conn->time_pings[0].remote_time;
uint64_t lt = conn->time_pings[0].local_time;
uint64_t tld = conn->time_pings[0].dispersion;
int chosen = 0;
for (cc = 1; cc < conn->time_ping_count; cc++)
if (conn->time_pings[cc].dispersion < tld) {
chosen = cc;
l2rtd = conn->time_pings[cc].local_to_remote_difference;
rt = conn->time_pings[cc].remote_time;
lt = conn->time_pings[cc].local_time;
tld = conn->time_pings[cc].dispersion;
// local_time_chosen = conn->time_pings[cc].local_time;
@@ -670,44 +663,8 @@ void *rtp_timing_receiver(void *arg) {
// dispersion.",chosen,1.0*((tld * 1000000) >> 32));
conn->time_pings[chosen].chosen = 1; // record the fact that it has been used for timing
/*
// calculate the jitter -- the absolute time between the current
local_to_remote_time_difference and the new one and add it to the total jitter count
int64_t ji;
int64_t ltd =0; // local time difference for the jitter
if (conn->time_ping_count > 1) {
if (l2rtd > conn->local_to_remote_time_difference) {
local_to_remote_time_jitter =
local_to_remote_time_jitter + l2rtd - conn->local_to_remote_time_difference;
ji = l2rtd - conn->local_to_remote_time_difference; // this is the difference
between the present local-to-remote-time-difference and the new one, i.e. the jitter
step
} else {
local_to_remote_time_jitter =
local_to_remote_time_jitter + conn->local_to_remote_time_difference - l2rtd;
ji = -(conn->local_to_remote_time_difference - l2rtd);
}
local_to_remote_time_jitter_count += 1;
}
if (conn->local_to_remote_time_difference_measurement_time < lt)
ltd = lt-conn->local_to_remote_time_difference_measurement_time;
else
ltd = -(conn->local_to_remote_time_difference_measurement_time-lt);
if (ltd) {
debug(1,"Jitter: %" PRId64 " nanoseconds in %" PRId64 " nanoseconds.", ji, ltd);
debug(1,"Source clock to local clock drift: %.2f ppm.",((1.0*ji)/ltd)*1000000.0);
}
// uncomment below to print jitter between client's clock and our clock
if (ji) {
debug(1,"Choosing time difference[%d] with dispersion of %" PRId64 " ns with an
adjustment of %" PRId64 " ns",chosen, tld, ji);
}
*/
conn->local_to_remote_time_difference =
l2rtd; // make this the new local-to-remote-time-difference
rt - lt; // make this the new local-to-remote-time-difference
conn->local_to_remote_time_difference_measurement_time = lt; // done at this time.
if (first_local_to_remote_time_difference == 0) {
@@ -730,57 +687,69 @@ void *rtp_timing_receiver(void *arg) {
int sample_count = 0;
// approximate time in seconds to let the system settle down
const int settling_time = 60;
const int settling_time = 30;
// number of points to have for calculating a valid drift
const int sample_point_minimum = 8;
for (cc = 0; cc < conn->time_ping_count; cc++)
if ((conn->time_pings[cc].chosen) &&
(conn->time_pings[cc].sequence_number >
(settling_time / 3))) { // wait for a approximate settling time
y_bar += (conn->time_pings[cc].remote_time >>
12); // precision is down to 1/4th of a microsecond
x_bar += (conn->time_pings[cc].local_time >> 12);
// have to scale them down so that the sum, possibly over every term in the array, doesn't overflow
y_bar += (conn->time_pings[cc].remote_time >> time_ping_history_power_of_two);
x_bar += (conn->time_pings[cc].local_time >> time_ping_history_power_of_two);
sample_count++;
}
if (sample_count > sample_point_minimum) {
y_bar = y_bar / sample_count;
x_bar = x_bar / sample_count;
int64_t xid, yid;
int64_t mtl, mbl;
double mtl, mbl;
mtl = 0;
mbl = 0;
for (cc = 0; cc < conn->time_ping_count; cc++)
if ((conn->time_pings[cc].chosen) &&
(conn->time_pings[cc].sequence_number > (settling_time / 3))) {
uint64_t slt = conn->time_pings[cc].local_time >> 12;
uint64_t slt = conn->time_pings[cc].local_time >> time_ping_history_power_of_two;
if (slt > x_bar)
xid = slt - x_bar;
else
xid = -(x_bar - slt);
uint64_t srt = conn->time_pings[cc].remote_time >> 12;
uint64_t srt = conn->time_pings[cc].remote_time >> time_ping_history_power_of_two;
if (srt > y_bar)
yid = srt - y_bar;
else
yid = -(y_bar - srt);
mtl = mtl + xid * yid;
mbl = mbl + xid * xid;
mtl = mtl + (1.0 * xid) * yid;
mbl = mbl + (1.0 * xid) * xid;
}
conn->local_to_remote_time_gradient_sample_count = sample_count;
if (mbl)
conn->local_to_remote_time_gradient = (1.0 * mtl) / mbl;
conn->local_to_remote_time_gradient = mtl / mbl;
else {
conn->local_to_remote_time_gradient = 1.0;
debug(1, "rtp_timing_receiver: mbl is 0");
}
// scale the numbers back up
uint64_t ybf = y_bar << time_ping_history_power_of_two;
uint64_t xbf = x_bar << time_ping_history_power_of_two;
conn->local_to_remote_time_difference =
ybf - xbf; // make this the new local-to-remote-time-difference
conn->local_to_remote_time_difference_measurement_time = xbf;
} else {
conn->local_to_remote_time_gradient = 1.0;
}
// debug(1,"local to remote time gradient is %12.2f ppm, based on %d
// samples.",conn->local_to_remote_time_gradient*1000000,sample_count);
} else {
debug(2,
"Time ping turnaround time: %" PRIu64