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I have been ripping my NTSC DVDs to MKV files using MakeMKV v1.9.9 Windows x64. Strangely, I see that several - but not all - files seem to have a different frame rate from the original. As these are NTSC DVDs I'm expecting all the frame rates to be 29.970 but MediaInfo is sometimes reporting 23.976. Here is an example of 'Dog Eat Dog' from CSI Season 6 episode 9.

Ripped using AnyDVD HD
General
Complete name : T:\TV shows\Drama\CSI\Season 06\Episodes 09-12\DVD_VIDEO\VIDEO_TS\VTS_02_1.VOB
Format : MPEG-PS
File size : 1 024 MiB
Duration : 25mn 14s
Overall bit rate mode : Variable
Overall bit rate : 5 671 Kbps

Video
ID : 224 (0xE0)
Format : MPEG Video
Format version : Version 2
Format profile : Main@Main
Format settings, BVOP : Yes
Format settings, Matrix : Custom
Format settings, GOP : M=3, N=12
Duration : 25mn 14s
Bit rate mode : Variable
Bit rate : 4 919 Kbps
Maximum bit rate : 9 800 Kbps
Width : 720 pixels
Height : 480 pixels
Display aspect ratio : 16:9
Frame rate : 29.970 (30000/1001) fps <----- original frame rate
Standard : NTSC
Color space : YUV
Chroma subsampling : 4:2:0
Bit depth : 8 bits
Compression mode : Lossy
Bits/(Pixel*Frame) : 0.475
Stream size : 888 MiB (87%)

Audio #1
ID : 189 (0xBD)-128 (0x80)
Format : AC-3
Format/Info : Audio Coding 3
Mode extension : CM (complete main)
Format settings, Endianness : Big
Muxing mode : DVD-Video
Duration : 25mn 14s
Bit rate mode : Constant
Bit rate : 448 Kbps
Channel(s) : 6 channels
Channel positions : Front: L C R, Side: L R, LFE
Sampling rate : 48.0 KHz
Frame rate : 31.250 fps (1536 spf)
Compression mode : Lossy
Stream size : 80.9 MiB (8%)

Audio #2
ID : 189 (0xBD)-129 (0x81)
Format : AC-3
Format/Info : Audio Coding 3
Format profile : Dolby Digital
Mode extension : CM (complete main)
Format settings, Endianness : Big
Muxing mode : DVD-Video
Duration : 25mn 14s
Bit rate mode : Constant
Bit rate : 192 Kbps
Channel(s) : 2 channels
Channel positions : Front: L R
Sampling rate : 48.0 KHz
Frame rate : 31.250 fps (1536 spf)
Compression mode : Lossy
Stream size : 34.7 MiB (3%)

Text
ID : 224 (0xE0)-CC3
Format : EIA-608
Muxing mode, more info : Muxed in Video #1
Bit rate mode : Constant
Stream size : 0.00 Byte (0%)

Menu

Ripped using MakeMKV
General
Unique ID : 162208426731755526576376653396381673446 (0x7A0834E589947204F0055BD78F8627E6)
Complete name : T:\TV shows\Drama\CSI\Season 06\09 - Dog Eat Dog.mkv
Format : Matroska
Format version : Version 2
File size : 1.59 GiB
Duration : 42mn 10s
Overall bit rate mode : Variable
Overall bit rate : 5 413 Kbps
Movie name : CSI: Season 6, Disc 3
Encoded date : UTC 2016-04-04 19:06:54
Writing application : MakeMKV v1.9.9 win(x64-release)
Writing library : libmakemkv v1.9.9 (1.3.3/1.4.4) win(x64-release)
Original source medium : DVD-Video

Video
ID : 1
ID in the original source medium : 224 (0xE0)
Format : MPEG Video
Format version : Version 2
Format profile : Main@Main
Format settings, BVOP : Yes
Format settings, Matrix : Custom
Format settings, GOP : M=3, N=12
Codec ID : V_MPEG2
Codec ID/Info : MPEG 1 or 2 Video
Duration : 42mn 10s
Bit rate mode : Variable
Bit rate : 4 960 Kbps
Maximum bit rate : 9 800 Kbps
Width : 720 pixels
Height : 480 pixels
Display aspect ratio : 16:9
Frame rate mode : Variable
Frame rate : 23.976 (23976/1000) fps<----- new frame rate
Original frame rate : 29.970 (30000/1001) fps
Color space : YUV
Chroma subsampling : 4:2:0
Bit depth : 8 bits
Compression mode : Lossy
Bits/(Pixel*Frame) : 0.599
Stream size : 1.46 GiB (92%)
Language : English
Default : No
Forced : No

Audio
ID : 2
ID in the original source medium : 189 (0xBD)-128 (0x80)
Format : AC-3
Format/Info : Audio Coding 3
Mode extension : CM (complete main)
Format settings, Endianness : Big
Codec ID : A_AC3
Duration : 42mn 10s
Bit rate mode : Constant
Bit rate : 448 Kbps
Channel(s) : 6 channels
Channel positions : Front: L C R, Side: L R, LFE
Sampling rate : 48.0 KHz
Frame rate : 31.250 fps (1536 spf)
Compression mode : Lossy
Stream size : 135 MiB (8%)
Title : Surround 5.1
Language : English
Default : Yes
Forced : No

Menu
00:00:00.000 : en:Chapter 01
00:02:10.997 : en:Chapter 02
00:08:59.005 : en:Chapter 03
00:20:30.028 : en:Chapter 04
00:29:45.984 : en:Chapter 05
00:41:43.701 : en:Chapter 06

Many DVDs were authored with 23.976 source material, because they were movies, or TV shows shot digitally at 23.976 in HD. In order make use of the limited space and try to provide a higher bitrate, the MPEG-2 video on the disc is 23.976 with repeat frames marked, so that when the MPEG-2 video is "converted" to analog video, it will be at the required 29.97 frame rate.

I'm sure that you know there is really no such thing as NTSC, or PAL, digital video. It is either 720x480 or 720x576. NTSC/PAL are ANALOG specifications. More so when the video on the disc is 23.976.

So, is there a problem with the video being at the original frame rate?

I don't have an obvious problem when playing the MKV files, regardless of whether MediaInfo shows them to have frame rates of 29.970 or 23.976. I'm just curious to know why MediaInfo shows an apparent change of frame rate for some files.

Which I explained.