-f flag followed by a selection string. For example, -f "bv*[ext=mp4]+ba[ext=m4a]/b[ext=mp4]/*" prioritizes the best video quality in MP4 format combined with M4A audio. This approach ensures you download the highest available resolution while maintaining compatibility with standard media players.
Understanding yt-dlp Format Selection Syntax
The core of downloading specific video qualities lies in understanding how yt-dlp parses format codes. Each video file on platforms like YouTube consists of separate streams for video and audio. By default, yt-dlp attempts to merge the best available video stream with the best available audio stream into a single container. However, this default behavior might not always yield the desired result if you have specific constraints regarding file extension or codec. You can inspect the available formats for any given video by using the--list-formats or -F option. This command outputs a table showing every available combination of resolution, frame rate, codec, and file extension. The output includes a numeric ID for each format, which serves as the reference point for your selection commands. Understanding this list is crucial because it reveals why certain resolutions might be missing or why a specific extension is preferred over another.
When constructing a selection string, you use comparison operators such as * (best), < (worse than), and [ext=mp4] or [height>=1080]. This granular control allows you to build complex queries that prioritize specific technical requirements over general quality metrics.
Specifying Maximum Resolution and Format
To explicitly target the maximum resolution, you should combine height filters with format selectors. A common pattern is to request the best video stream that meets a minimum height requirement, paired with the best audio stream. For instance, if you want 1080p or higher, you might use:yt-dlp -f "bv*[height>=1080][ext=mp4]+ba[ext=m4a]/b[height>=1080][ext=mp4]/*"bv* selects the best video-only stream, filtered by height greater than or equal to 1080 pixels and an MP4 extension. The +ba part appends the best audio-only stream in M4A format. If no such combined format exists, the fallback /b... section attempts to find a single bundled stream that matches the criteria.
If you simply want the absolute highest resolution regardless of other factors, you can use -f bv/b. Here, bv stands for best video only, and b is the fallback to best overall. To force a specific format like MKV, you would change the extension filter to [ext=mkv]. It is important to note that not all platforms support every container format for high-resolution streams. For example, some services may only offer 4K content in WebM format, making MP4 selection impossible at that resolution.
Handling Merge Failures and Post-Processing
Sometimes, selecting a specific format leads to issues where the downloaded files do not merge correctly. This often happens when the video and audio codecs are incompatible with the chosen container. In such cases, yt-dlp may leave you with two separate files instead of one merged video. To address this, you can instruct yt-dlp to post-process the file using FFmpeg. Adding the--merge-output-format mp4 flag forces yt-dlp to remux the final output into an MP4 container after downloading the individual streams. This step is essential if you have selected disparate formats that cannot be naturally muxed. However, remuxing takes additional time and CPU resources. If you encounter seekability issues in browsers after downloading, refer to guides on fixing dynamic resolution video issues or ensuring FFmpeg outputs seekable files.
Another consideration is the order of operations. yt-dlp evaluates the format selection string from left to right. Therefore, placing your most preferred format first ensures it is attempted before falling back to less ideal options. This hierarchical approach prevents accidental downloads of lower-quality streams when higher-quality alternatives are available but slightly more complex to select.
For users who frequently mix audio channels or need precise control over the final output, understanding these selection strings is vital. Incorrect selections can lead to videos that look correct but have audio synchronization problems or incorrect channel layouts. Always verify the output format after download to ensure it matches your expectations.
Optimizing Downloads for Specific Use Cases
Different use cases require different optimization strategies. If you are archiving content, you might prefer lossless formats or raw streams. If you are preparing content for web playback, smaller file sizes and broad compatibility become priorities. You can tailor your commands accordingly by adjusting the extension filters and codec preferences. For mobile devices, you might restrict the height toheight<=720 to save storage space. Conversely, for desktop viewing, you might demand height>=2160 for 4K content. Remember that higher resolutions often come with significantly larger file sizes and longer download times. Balancing quality with bandwidth constraints is a key part of effective usage.
Additionally, consider using configuration files to store your favorite format strings. This avoids typing long commands repeatedly. You can define aliases in your shell profile or use yt-dlp's built-in config file support to set default parameters. This streamlines your workflow and reduces the chance of typos in complex format strings.
For related tasks, such as joining images with video frames or mixing audio channels, the principles of format selection remain similar. Consistency in your approach helps maintain quality across different types of media processing tasks. Always test your commands on small clips before applying them to large archives to avoid wasting time on failed merges.
Frequently Asked Questions
Why does yt-dlp download two separate files instead of one?
This occurs when the best video and best audio streams use incompatible codecs or containers that cannot be natively muxed. To fix this, add the --merge-output-format mp4 flag to force FFmpeg to remux them into a single file.
How can I force yt-dlp to download only 1080p and not 4K?
You can limit the maximum height by using -f "bv*[height<=1080]". This tells yt-dlp to ignore any video streams with a height greater than 1080 pixels, effectively capping the resolution.
What is the difference between bv and b in format strings?
bv selects the best video-only stream, while b selects the best overall format, which may include both video and audio in a single file. Using bv+ba combines the best video and best audio separately, often resulting in higher quality than the bundled b.
Can I use yt-dlp to download subtitles along with the video?
Yes, you can add --write-subs --sub-lang en to your command. This will download English subtitles and embed them into the video file or save them as separate .vtt files depending on your configuration.
Why are my downloaded videos not seekable in Chrome or Edge?
This issue often arises from improper container headers or fragmented MP4 files. Ensure you are using --merge-output-format mp4 or converting the file with FFmpeg afterward to create a standard, seekable MP4 structure.