Clarify statistics doc

The descriptions of Net Sync PPM and All Sync PPM appeared to be partly
swapped. Also generally clarify them and harmonize the wording with the
introduction.

Author: willmo

ADVANCED TOPICS/Statistics.md

@@ -2,7 +2,7 @@
 
 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.
+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 in the last interval, i.e. the number of frames added plus the number of frames removed from the audio stream, relative to the total number of frames output, 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. 
 
@@ -16,9 +16,9 @@ Here is a brief description of the figures that might be provided.
 ##### Sync Error ms
 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.
 ##### Net Sync PPM
-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 in the last interval, i.e. the number of frames added **minus** the number of frames removed from the audio stream, relative to the total number of frames output, 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.
 ##### All Sync 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 total amount of interpolation done by Shairport Sync to keep the audio stream in sync in the last interval, i.e. the number of frames added **plus** the number of frames removed from the audio stream, relative to the total number of frames output, expressed in parts per million (PPM). 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.
 ##### Packets
 This is the number of packets of audio frames received since the start of the session. A packet normally contains 352 ± 1 audio frames.
 ##### Missing