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I am doing some transcoding via ffmpeg and am noticing video and audio FPS differing greatly. During playback there is no issue, everything is working as expected. So I am trying to understand what it means for audio to be at much higher framerate and how audio is not going out of sync due to that?

Thank you.

Video
ID                                       : 1
Format                                   : AVC
Format/Info                              : Advanced Video Codec
Format profile                           : High@L3
Format settings                          : CABAC / 4 Ref Frames
Format settings, CABAC                   : Yes
Format settings, ReFrames                : 4 frames
Codec ID                                 : V_MPEG4/ISO/AVC
Duration                                 : 46 min 12 s
Width                                    : 852 pixels
Height                                   : 480 pixels
Display aspect ratio                     : 16:9
Frame rate mode                          : Constant
Frame rate                               : 23.976 (24000/1001) FPS
Color space                              : YUV
Chroma subsampling                       : 4:2:0
Bit depth                                : 8 bits
Scan type                                : Progressive
Writing library                          : x264 core 152 r2854 e9a5903
Encoding settings                        : cabac=1 / ref=1 / deblock=1:0:0 / analyse=0x3:0x113 / me=hex / subme=2 / psy=1 / psy_rd=1.00:0.00 / mixed_ref=0 / me_range=16 / chroma_me=1 / trellis=0 / 8x8dct=1 / cqm=0 / deadzone=21,11 / fast_pskip=1 / chroma_qp_offset=0 / threads=9 / lookahead_threads=3 / sliced_threads=0 / nr=0 / decimate=1 / interlaced=0 / bluray_compat=0 / constrained_intra=0 / bframes=3 / b_pyramid=2 / b_adapt=1 / b_bias=0 / direct=1 / weightb=1 / open_gop=0 / weightp=1 / keyint=250 / keyint_min=23 / scenecut=40 / intra_refresh=0 / rc_lookahead=10 / rc=crf / mbtree=1 / crf=23.0 / qcomp=0.60 / qpmin=0 / qpmax=69 / qpstep=4 / ip_ratio=1.40 / aq=1:1.00
Default                                  : Yes
Forced                                   : No

Audio
ID                                       : 2
Format                                   : AAC
Format/Info                              : Advanced Audio Codec
Format profile                           : LC
Codec ID                                 : A_AAC-2
Duration                                 : 46 min 12 s
Channel(s)                               : 2 channels
Channel positions                        : Front: L R
Sampling rate                            : 48.0 kHz
Frame rate                               : 46.875 FPS (1024 SPF)
Compression mode                         : Lossy
Delay relative to video                  : -104 ms
Writing library                          : Lavc57.107.100 aac
Language                                 : English
Default                                  : Yes
Forced                                   : No

1 Answer 1

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Audio data consists of samples. The sampling rate indicates the number of samples per channel in one second. But when dealing with audio, applications read and process samples in chunks. These chunks are the frames. The AAC encoder uses 1024 Samples Per Frame. So, the audio "Frame Rate" is Sampling rate/1024 for AAC audio. This is not related to video frame rate.

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