If you set the statistics setting in the diagnostics section of the configuration file to "YES", some statistics will be logged at regular intervals. The items logged will depend on the type of stream being processed: classic AirPlay, AirPlay 2 Buffered Audio or AirPlay 2 Realtime Audio. If the log_verbosity is set to 1, 2 or 3, additional items will be logged.
From an audio enthusiast's point of view, the most important figure is possibly the All Sync PPM figure. This is the total amount of interpolation needed by Shairport Sync to keep the audio stream in sync. The number represents is the ratio of frames added and removed from the audio stream relative to all the frames output in the last interval, expressed in parts per million (PPM). For reference, adding or removing one frame per second into a 44,100 frames per second stream is ± 22.68 PPM. The lower this number number is, the higher the fidelity of the audio signal passed to the output device. On a well sorted system, this figure can be 0.0 for considerable periods, but it can't really be zero forever unless the output device is adapting to the true data rate (some very high-end streamers seem to do so). You may also find that the number might be higher at the start while the system settles down.
The second most important figure is possibly the Sync Error ms. This is the average synchronisation error in milliseconds in the last interval. Ideally it should be 0.0. By default, Shairport Sync has a tolerance of a sync error of ± 2.0 milliseconds without triggering interpolation.
Two other interesting measurements of the output rate may be available – Output FPS (r) and Output FPS (c), where (r) means "raw" and the (c) means "corrected".
The "raw" figure is the rate at which the output device (typically a DAC) accepts data measured relative to the computer's own system clock (specifically the CLOCK_MONOTONIC_RAW clock). The accuracy of the number depends on the accuracy of the clock, which will typically be accurate to within anything from 20 to 100 ppm.
The "corrected" figure is the rate at which the output device accepts data relative to the computer's network-time-disciplined clock (specifically the CLOCK_MONOTONIC clock). This clock is normally adjusted ("disciplined") to keep time with network time and should be accurate to with a few tens of milliseconds over a long period. So (1) if you could run a play session for a long period – say a day – and (2) if network time synchronisation is enabled and (3) if the network connection to the network time service is fast and stable, then you should get an accurate absolute measure of exact frame rate of the output device. If your internet connection is not good, the corrected figure will be very inaccurate indeed.
Here is a brief description of the figures that might be provided.
Average playback synchronisation error in milliseconds in the last interval. By default, Shairport Sync will allow a sync error of ± 2.0 milliseconds without any interpolation. Positive means late, negative means early.
This is the total amount of interpolation done by Shairport Sync to keep the audio stream in sync. The number represents is the total number of frames added and removed from the audio stream, expressed in parts per million (PPM) in the last interval. For reference, adding or removing one frame per second into a 44,100 frames per second stream is 22.68 ppm.
This is the net amount of interpolation done by Shairport Sync to keep the audio stream in sync. The number represents is the number of frames added plus the number removed from the audio stream, expressed in parts per million (PPM) in the last interval. The magnitude of this should be the same as the net sync ppm. If it is much larger it means that Shairport Sync is overcorrecting for sync errors – try increasing the drift tolerance to reduce it.
This is the number of packets of audio frames received since the start of the session. A packet normally contains 352 ± 1 audio frames.
This is the number of packets of audio frames that were expected but not received in the last interval. It should be zero, and if not it usually indicates a significant problem with the network. classic AirPlay and AirPlay 2 Realtime Streams only.
This is the number of packets of audio frames that were received late – but still in time to be used – in the last interval. Classic AirPlay and AirPlay 2 Realtime Streams only.
This is the number of packets of audio frames that were received too late to be used in the last interval. It is possible that these packets were already received, so those frames might not actually be missing when the time comes to play them. Classic AirPlay and AirPlay 2 Realtime Streams only.
This is the number of times Shairport Sync requests the resending of missing frames. Requests can be for one or more frames. Classic AirPlay and AirPlay 2 Realtime Streams only.
The is the smallest number of frames of audio in the DAC's hardware queue. If it goes too low, the DAC may begin to underrun.
The is the smallest number of packets of audio in the queue to be processed in the last interval. It is related to the overall latency in Classic AirPlay and AirPlay 2 Realtime Streams. If it comes close to zero it's often a sign that the network is poor.
The is the largest number of packets of audio in the queue to be processed in the last interval.
The is smallest remaining number of bytes in the Buffered Audio buffer in the last interval. It can legitimately be zero when a track ends or begins. If it reaches zero while a track is playing, it means that audio data is not arriving at Shairport Sync quickly enough and may indicate network problems. AirPlay 2 Buffered Audio streams only.
This is the rate specified in the AirPlay stream itself. Classic AirPlay only.
This is the rate at which frames are received from the network averaged since the start of the play session. Classic AirPlay only.
Output rate measured relative to the computer system's clock since the start of the play session. See above for a discussion.
Output rate measured relative to the network-clock-disciplined computer system's clock since the start of the play session. See above for a discussion.
This is a measure of the difference between the source clock and Shairport Sync's clock expressed in parts per million. Only valid when 10 or more drift samples have been received. Classic AirPlay only.
This is the number drift samples have been accepted for calculating the source drift. Classic AirPlay only.
The following example is of an AirPlay 2 Buffered Audio Stream from a HomePod mini to a WiFi-connected Raspberry Pi 3 equipped with a Pimoroni "Audio DAC SHIM (Line-Out)" with log_verbosity set to 1 after 3 hours and 37 minutes of operation. The audio is from a radio station accessed through Apple Music:
5.292055362 "rtsp.c:3112" Connection 1. AP2 Buffered Audio Stream.
1.382815677 "player.c:2650" Connection 1: Playback Started -- AirPlay 2 Buffered.
7.899808955 "player.c:2491" Sync Error ms | Net Sync PPM | All Sync PPM | Min DAC Queue | Min Buffer Size | Output FPS (r) | Output FPS (c)
...
8.014025361 "player.c:2491" -1.69 2.8 2.8 7341 198k 44100.04 44100.35
7.992082237 "player.c:2491" -1.70 2.8 2.8 7332 188k 44100.04 44100.34
8.015987549 "player.c:2491" -1.68 0.0 0.0 7334 186k 44100.04 44100.35
8.010537393 "player.c:2491" -1.68 2.8 2.8 7335 187k 44100.04 44100.36
7.993940934 "player.c:2491" -1.85 0.0 0.0 7329 198k 44100.04 44100.37
8.015796195 "player.c:2491" -1.87 5.7 5.7 7325 194k 44100.04 44100.37
8.021665934 "player.c:2491" -1.92 5.7 5.7 7329 190k 44100.04 44100.37
7.990409477 "player.c:2491" -1.85 2.8 2.8 7347 201k 44100.04 44100.37
8.001858278 "player.c:2491" -1.96 17.0 17.0 7332 204k 44100.04 44100.37
8.007150986 "player.c:2491" -1.89 2.8 2.8 7327 195k 44100.04 44100.36
7.998612862 "player.c:2491" -1.87 2.8 2.8 7317 199k 44100.04 44100.36
8.020592653 "player.c:2491" -1.89 11.3 11.3 7323 203k 44100.04 44100.35
8.003245674 "player.c:2491" -1.89 8.5 8.5 7325 198k 44100.04 44100.35
7.990874789 "player.c:2491" -1.98 19.8 19.8 7103 197k 44100.04 44100.34
8.010600257 "player.c:2491" -1.85 2.8 2.8 7327 187k 44100.04 44100.33
8.004573122 "player.c:2491" -1.89 5.7 5.7 7327 187k 44100.04 44100.33
8.012853278 "player.c:2491" -1.89 5.7 5.7 7326 190k 44100.04 44100.32
8.005596664 "player.c:2491" -1.95 22.7 22.7 7322 165k 44100.04 44100.31
8.019198695 "player.c:2491" -1.85 5.7 5.7 7333 166k 44100.04 44100.31
7.999363382 "player.c:2491" -1.79 2.8 2.8 7349 153k 44100.04 44100.30
7.990332549 "player.c:2491" -1.87 5.7 5.7 7328 155k 44100.04 44100.29
8.012287445 "player.c:2491" -1.84 2.8 2.8 7352 185k 44100.04 44100.28
7.998929528 "player.c:2491" -1.94 14.2 14.2 7331 171k 44100.04 44100.28
8.008170622 "player.c:2491" -1.88 5.7 5.7 7327 188k 44100.04 44100.27
8.012935257 "player.c:2491" -1.87 19.8 19.8 7270 205k 44100.04 44100.26
8.006337966 "player.c:2491" -1.86 0.0 0.0 7334 199k 44100.04 44100.25
8.004206612 "player.c:2491" -1.76 2.8 2.8 7331 204k 44100.04 44100.25
7.998495257 "player.c:2491" -1.82 2.8 2.8 7329 204k 44100.04 44100.24
8.016859059 "player.c:2491" -1.88 2.8 2.8 7335 201k 44100.04 44100.23
7.993529320 "player.c:2491" -1.92 14.2 14.2 7329 205k 44100.04 44100.22
8.021956820 "player.c:2491" -1.96 11.3 11.3 7322 206k 44100.04 44100.21
8.006401820 "player.c:2491" -1.93 11.3 11.3 7322 199k 44100.04 44100.21
8.001469111 "player.c:2491" -1.89 5.7 5.7 7336 204k 44100.04 44100.20
7.995194320 "player.c:2491" -1.86 5.7 5.7 7330 202k 44100.04 44100.19
8.017805206 "player.c:2491" -1.99 22.7 22.7 7322 201k 44100.04 44100.18
7.995541038 "player.c:2491" -1.95 22.7 22.7 7331 203k 44100.04 44100.18
8.011463643 "player.c:2491" -1.87 0.0 0.0 7331 207k 44100.04 44100.17
7.995941403 "player.c:2491" -1.85 2.8 2.8 7331 206k 44100.04 44100.16
8.013330570 "player.c:2491" -1.71 2.8 2.8 7344 212k 44100.04 44100.15
8.012214580 "player.c:2491" -1.82 0.0 0.0 7333 203k 44100.04 44100.15
7.998585049 "player.c:2491" -1.85 8.5 8.5 7332 207k 44100.04 44100.14
8.023448799 "player.c:2491" -1.86 5.7 5.7 7328 206k 44100.04 44100.13
7.984586612 "player.c:2491" -1.87 8.5 8.5 7277 205k 44100.04 44100.12
8.017339372 "player.c:2491" -1.91 17.0 17.0 7327 207k 44100.04 44100.12
8.007090309 "player.c:2491" -1.94 17.0 17.0 7286 199k 44100.04 44100.11
8.007894164 "player.c:2491" -1.87 2.8 2.8 7334 201k 44100.04 44100.10
7.991787497 "player.c:2491" -1.89 2.8 2.8 7327 194k 44100.04 44100.09
8.019011611 "player.c:2491" -1.94 5.7 5.7 7323 185k 44100.04 44100.09
7.994695362 "player.c:2491" -1.99 14.2 14.2 7321 199k 44100.04 44100.08
8.010284632 "player.c:2491" -1.93 11.3 11.3 7325 191k 44100.04 44100.07
8.011451612 "player.c:2491" -1.87 8.5 8.5 7329 156k 44100.04 44100.09
7.993112549 "player.c:2491" -1.84 2.8 2.8 7330 150k 44100.04 44100.13
8.004971455 "player.c:2491" -1.77 11.3 11.3 7325 157k 44100.04 44100.16
8.010517133 "player.c:2491" -1.85 0.0 0.0 7322 182k 44100.04 44100.19
8.005598851 "player.c:2491" -2.00 39.7 39.7 7207 180k 44100.04 44100.22
8.006777965 "player.c:2491" -1.99 39.7 39.7 7320 183k 44100.04 44100.24
7.115508696 "player.c:1643" Connection 1: Playback Stopped. Total playing time 03:37:20. Output: 44100.04 (raw), 44100.24 (corrected) frames per second.
For reference, a drift of one second per day is approximately 11.57 ppm. Left uncorrected, even a drift this small between two audio outputs will be audible after a short time.