Test log: podcast-on-spotify guide. Run 2026-10-25 by Koldflux on Ubuntu, ffmpeg/ffprobe 6.1.1, Playwright headless Chromium. 1. Test episode: 6 sentences of our own text, synthetic voice (Kokoro-82M, Apache-2.0), 48 kHz stereo 16-bit WAV master. codec_name=pcm_s16le sample_rate=48000 channels=2 duration=27.702667 size=5318990 2. Live Koldflux tool https://koldflux.com/tools/convert/audio/wav-to-mp3 driven in headless Chromium with that WAV (setInputFiles, Convert, Download). Output ffprobe: codec_name=mp3 sample_rate=48000 channels=2 bit_rate=128000 duration=27.744000 size=444141 Source WAV size 5318990 bytes -> MP3 444141 bytes. Sample rate and channels kept. Audio present. 3. Live Koldflux tool https://koldflux.com/tools/subtitles/srt-to-vtt with our SRT (6 cues). Result diffed against the VTT we generated from the same timings: byte-identical (diff exit 0). Size 588 bytes, limit stated by Spotify is 5 MB. 4. Spec checker (spotify_check.py, printed on the page) against Spotify help specs read 2026-10-25. --- tool-out.mp3 (audio file) PASS Audio file type is MP3, M4A or WAV [.mp3] PASS Mono or stereo [2 channel(s)] INFO Codec / sample rate / bitrate [mp3, 48000 Hz, 128 kbps] INFO File size (no limit stated; large files can fail on a bad connection) [0.4 MB] RESULT OK (WARN lines are recommendations, not errors) --- episode-1080p.mp4 (cover frame + MP3 audio, x264 High, 25 Mbit/s CBR, 30 fps, keyframe every 1 s, AAC-LC 192k, bt709) PASS Container MP4 (recommended) or MOV [.mp4] PASS Exactly one video track and one audio track [1 video, 1 audio] PASS Video codec H.264 (H.265 also compatible) [h264 High] PASS Resolution 1080p or higher (recommended) [1920x1080] PASS Aspect ratio 16:9 (others are letterboxed) [1.778] PASS Frame rate 24, 25, 30, 50 or 60 [30.00] PASS Progressive scan [progressive] PASS 4:2:0 color sampling, 8 or 10 bit (recommended) [yuv420p] PASS Color space Rec.709 (recommended; INFO = not tagged in the file) [bt709] PASS Audio AAC-LC (recommended) [aac LC] PASS Audio 128 kbps or higher (192+ in the upload article) [176 kbps] PASS Stereo (mono is upmixed, 5.1 downmixed) [2 ch] PASS Audio and video tracks have the same duration [video 27.70s, audio 27.70s] PASS Duration under 4 hours (recommended), 12 hours at most [27.7 s] PASS File size under 10 GB (recommended), 60 GB at most [86.9 MB] PASS Keyframe about every 1 s (recommended), at least one per 60 s [28 keyframes, longest gap 1.00 s] RESULT OK (WARN lines are recommendations, not errors) --- episode-1080p-crf.mp4 (same, CRF 18) PASS Container MP4 (recommended) or MOV [.mp4] PASS Exactly one video track and one audio track [1 video, 1 audio] PASS Video codec H.264 (H.265 also compatible) [h264 High] PASS Resolution 1080p or higher (recommended) [1920x1080] PASS Aspect ratio 16:9 (others are letterboxed) [1.778] PASS Frame rate 24, 25, 30, 50 or 60 [30.00] PASS Progressive scan [progressive] PASS 4:2:0 color sampling, 8 or 10 bit (recommended) [yuv420p] PASS Color space Rec.709 (recommended; INFO = not tagged in the file) [bt709] PASS Audio AAC-LC (recommended) [aac LC] PASS Audio 128 kbps or higher (192+ in the upload article) [176 kbps] PASS Stereo (mono is upmixed, 5.1 downmixed) [2 ch] PASS Audio and video tracks have the same duration [video 27.70s, audio 27.70s] PASS Duration under 4 hours (recommended), 12 hours at most [27.7 s] PASS File size under 10 GB (recommended), 60 GB at most [1.8 MB] PASS Keyframe about every 1 s (recommended), at least one per 60 s [28 keyframes, longest gap 1.00 s] RESULT OK (WARN lines are recommendations, not errors) --- bad.mp4 (negative control: 720x1280 portrait, 29.97 fps, mono AAC 96k, one keyframe per 10 s) PASS Container MP4 (recommended) or MOV [.mp4] PASS Exactly one video track and one audio track [1 video, 1 audio] PASS Video codec H.264 (H.265 also compatible) [h264 High] WARN Resolution 1080p or higher (recommended) [720x1280] WARN Aspect ratio 16:9 (others are letterboxed) [0.562] PASS Frame rate 24, 25, 30, 50 or 60 [29.97] PASS Progressive scan [progressive] PASS 4:2:0 color sampling, 8 or 10 bit (recommended) [yuv420p] INFO Color space Rec.709 (recommended; INFO = not tagged in the file) [unset] PASS Audio AAC-LC (recommended) [aac LC] WARN Audio 128 kbps or higher (192+ in the upload article) [93 kbps] WARN Stereo (mono is upmixed, 5.1 downmixed) [1 ch] PASS Audio and video tracks have the same duration [video 9.98s, audio 10.00s] PASS Duration under 4 hours (recommended), 12 hours at most [10.0 s] PASS File size under 10 GB (recommended), 60 GB at most [0.1 MB] WARN Keyframe about every 1 s (recommended), at least one per 60 s [1 keyframes, longest gap 10.00 s] RESULT OK (WARN lines are recommendations, not errors) File sizes: CBR 25 Mbit/s MP4 86896982 bytes for 27.7 s; CRF 18 MP4 1819989 bytes. Not tested: an actual upload to Spotify for Creators (we use no account). Acceptance by the uploader is not shown by this log.