Convert Progressive to Interlaced with FFmpeg tinterlace

Converting progressive video to an interlaced format is a common requirement for television broadcasting and compatibility with legacy analog systems. This article provides a straightforward guide on how to use FFmpeg’s tinterlace (temporal interlace) filter to achieve this conversion using temporal decimation. You will learn the correct command-line syntax, how the filter processes video frames, and how to flags the output file correctly for interlaced playback.

Understanding Temporal Decimation with tinterlace

Temporal decimation in this context involves taking a high-frame-rate progressive video (such as 60p or 50p) and merging successive frames into alternating fields of a standard-frame-rate interlaced video (such as 30i or 25i).

For example, a 60 frames-per-second progressive video contains 60 full frames. By applying the tinterlace filter, FFmpeg takes Frame 1 and extracts its odd lines to create Field 1, then takes Frame 2 and extracts its even lines to create Field 2. These two fields are combined into a single interlaced frame. This process halves the overall frame rate (from 60fps to 30fps) while preserving the temporal smoothness of the original 60fps capture.

The FFmpeg Command Syntax

To convert a progressive video to interlaced using temporal decimation, use the following basic command structure:

ffmpeg -i input_60p.mp4 -vf "tinterlace=mode=interlaceme" -flags +ilme+ildct output_30i.mp4

Command Breakdown:

• -i input_60p.mp4: Specifies your high-frame-rate progressive source video.
• -vf "tinterlace=mode=interlaceme": Applies the video filter. interlaceme (or mode=4) takes two consecutive progressive frames and merges them into one interlaced frame, resulting in a frame rate reduction by half.
• -flags +ilme+ildct: These flags are critical for the encoder.
  • ilme forces interlaced motion estimation.
  • ildct forces interlaced discrete cosine transform (DCT) support. Without these flags, the encoder might compress the video as progressive, causing visual artifacts during playback.

Specifying Field Order (TFF vs. BFF)

Interlaced video relies on a specific field order: Top Field First (TFF) or Bottom Field First (BFF). By default, tinterlace outputs Top Field First.

If your target delivery format requires Bottom Field First (BFF), you must specify this in the filter arguments:

ffmpeg -i input_60p.mp4 -vf "tinterlace=mode=interlaceme:flags=blackman" -flags +ilme+ildct output_30i.mp4

To explicitly set the field order output for downstream devices, you can chain the fieldorder filter:

For Top Field First (TFF):

ffmpeg -i input_60p.mp4 -vf "tinterlace=mode=interlaceme,fieldorder=tff" -flags +ilme+ildct output_30i_tff.mp4

For Bottom Field First (BFF):

ffmpeg -i input_60p.mp4 -vf "tinterlace=mode=interlaceme,fieldorder=bff" -flags +ilme+ildct output_30i_bff.mp4

Verifying the Interlaced Output

After running the command, you can verify that the output video is correctly recognized as interlaced by using ffprobe:

ffprobe -v error -select_streams v:0 -show_entries stream=field_order,r_frame_rate input_30i.mp4

The output should display the halved frame rate (e.g., 30/1 or 2997/100 instead of 60/1) and the specified field order (tb for Top First, or bt for Bottom First).