| ---
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|
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| license: cc-by-sa-4.0
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| pretty_name: Captioned MIDI with ChordStream
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| task_categories:
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| - text-to-audio
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| tags:
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| - midi
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| - music
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| - captions
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| - chords
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| - symbolic-music
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| configs:
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| - config_name: default
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| data_files:
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| - split: train
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| path: data/train-*
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| - split: test
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| path: data/test-*
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| dataset_info:
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| features:
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| - name: i
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| dtype: int64
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| - name: source
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| dtype:
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| class_label:
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| names:
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| '0': godzilla
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| '1': midicaps_prose
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| '2': pdmx
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| - name: caption
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| dtype: string
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| - name: key_tok
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| dtype: string
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| - name: midi_name
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| dtype: string
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| - name: midi
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| dtype: binary
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| - name: chordstream
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| sequence: uint64
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| - name: cs_tokens
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| dtype: int32
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| - name: cs_bars
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| dtype: int32
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| - name: cs_key
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| dtype: string
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| - name: cs_error
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| dtype: string
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| splits:
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| - name: train
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| num_bytes: 35220596
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| num_examples: 900
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| - name: test
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| num_bytes: 3718590
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| num_examples: 100
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| download_size: 18872022
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| dataset_size: 38939186
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| ---
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|
|
| # Captioned MIDI with ChordStream
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|
|
| 1,000 captioned MIDI files, each with its harmony extracted as a **ChordStream** token
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| sequence: one 64-bit token per chord change or silence, carrying the key, the chord's
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| degree in that key, its pitch-class content, its bass, and its absolute bar, onset and
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| length in musical time.
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|
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| Rows are the first 1,000 `midicaps_prose` rows of `PsiPi/captioned-midi-moods-and-genre`
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| (a private dataset), with the ChordStream column computed from each row's own `midi` bytes.
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|
|
| ## Splits
|
|
|
| | split | rows |
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| |---|---|
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| | train | 900 |
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| | test | 100 |
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|
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| Split 90/10 with `train_test_split(test_size=0.1, seed=0)`.
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|
|
| ## Columns
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|
|
| | column | meaning |
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| |---|---|
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| | `i` | row id in the source dataset |
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| | `source` | always `midicaps_prose` here |
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| | `caption` | the prose caption |
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| | `key_tok` | key as labelled in the source dataset |
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| | `midi_name` | `{i}-{original file name}.mid` |
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| | `midi` | the MIDI file's bytes |
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| | `chordstream` | the ChordStream tokens, `uint64` per token |
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| | `cs_tokens` | number of tokens |
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| | `cs_bars` | bars covered |
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| | `cs_key` | key at the start of the piece, e.g. `C`, `Am`; the key can change, and each token carries its own |
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| | `cs_error` | empty, or why extraction produced nothing |
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|
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| Every token's top bit is zero, so the column is equally safe read as `uint64` or `int64`.
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| That is deliberate: numpy promotes `uint64` above 2^63-1 to float64 whenever it meets a
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| signed type, and Arrow refuses such values outright.
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|
|
| ## ChordStream token layout
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|
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| A token is two 32-bit words: `word_a = token & 0xFFFFFFFF`, `word_b = token >> 32`.
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|
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| **Word A (chord)**
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|
|
| | bits | field | meaning |
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| |---|---|---|
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| | 0-3 | tonic | key tonic pitch class 0-11 |
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| | 4 | mode | 0 major, 1 minor |
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| | 5-8 | degree | chord root relative to the tonic 0-11; **14 = solo line**, **15 = silence** |
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| | 9-19 | mask | bit i-1 set means the interval i semitones above the root is present |
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| | 20-23 | bass | bass pitch class relative to the chord root |
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| | 24-31 | reserved | zero |
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|
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| **Word B (time)**
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|
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| | bits | field | meaning |
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| |---|---|---|
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| | 0-15 | bar | absolute bar from the start of the piece |
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| | 16-21 | onset | sixteenths after the bar's downbeat |
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| | 22-29 | dur | length in sixteenths (1-255) |
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| | 30 | cont | 1 = continuation of the previous token (a chord that began earlier) |
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| | 31 | reserved | zero (the token's top bit) |
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|
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| The absolute chord root is `(tonic + degree) mod 12`. Everything except the tonic is
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| relative to the key, so transposing a piece changes only the tonic. Time is in sixteenths
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| derived from ticks, never from tempo, so playback speed cannot change a token. Every token
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| carries its own bar, onset and key, so any contiguous slice is self-describing: a window
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| whose first token has `bar > 0` began mid-piece, and its `cont` flag says whether its
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| chord started earlier.
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|
|
| ## Reading it
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|
|
| ```python
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| from datasets import load_dataset
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|
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| PC = ["C", "Db", "D", "Eb", "E", "F", "Gb", "G", "Ab", "A", "Bb", "B"]
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| ROMAN = ["I", "bII", "II", "bIII", "III", "IV", "bV", "V", "bVI", "VI", "bVII", "VII"]
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|
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| ds = load_dataset("PsiPi/captioned-ChordStream", split="train")
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| row = ds[0]
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|
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| for tok in row["chordstream"]:
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| a, b = tok & 0xFFFFFFFF, tok >> 32
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| tonic, mode = a & 15, a >> 4 & 1
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| degree, mask, bass = a >> 5 & 15, a >> 9 & 0x7FF, a >> 20 & 15
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| bar, onset, dur, cont = b & 0xFFFF, b >> 16 & 63, b >> 22 & 255, b >> 30 & 1
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|
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| if degree == 15:
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| name = "N.C." # silence
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| elif degree == 14:
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| name = "solo" # a single line, no harmony to name
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| else:
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| root = (tonic + degree) % 12
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| minorish = mask >> 2 & 1 and not mask >> 3 & 1 # minor third, no major third
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| numeral = ROMAN[degree].lower() if minorish else ROMAN[degree]
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| intervals = [i for i in range(1, 12) if mask >> (i - 1) & 1]
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| name = f"{numeral} {PC[root]} intervals={intervals} bass=+{bass}"
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| print(f"bar {bar}.{onset} dur {dur}{' cont' if cont else ''} "
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| f"key {PC[tonic]}{'m' if mode else ''} {name}")
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| ```
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|
|
| ## How the harmony was extracted
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|
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| Per chord step (the metre's beat, grouped to at least a quarter note), a duration-weighted
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| pitch-class histogram is matched against 16 chord templates, with melody voices
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| downweighted, pedal points removed and refitted, unaccompanied single lines marked as
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| solo, passing chords absorbed and consecutive same-root steps merged. The key comes from
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| Aarden-Essen profiles over a sliding 8-bar window with hysteresis. The full procedure,
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| with every threshold, is in the ChordStream write-up (see **Citation and further reading**).
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|
|
| ## Known limitations
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|
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| - **The extractor is heuristic** and has not been validated against human harmonic
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| annotation. Chord quality is the noisy part: power chords, add9 and sus2 are
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| over-produced where melody notes leak into the chord. Roots and cadences are more
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| reliable.
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| - **Only major and minor keys** are modelled.
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| - **3 of the 1,000 rows produced no tokens** (`cs_error` is `no pitched notes`: the file
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| has drums only or no notes the extractor uses). They are kept, with empty token lists,
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| so rows stay aligned with the source dataset.
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| - **Expressive performances** (a recorded performance with a dense tempo map rather than
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| a quantised score) have nominal bar lines, so chord steps placed on them may not match
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| the harmonic rhythm a listener hears. This has not been measured.
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|
|
| ## Licence and provenance
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|
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| The **MIDI files** come from the `midicaps_prose` portion of the parent dataset, which is
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| [MidiCaps](https://huggingface.co/datasets/amaai-lab/MidiCaps) over the Lakh MIDI dataset:
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| **CC BY-SA 4.0**.
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|
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| The **captions are not MidiCaps captions**. They were written for this project by PsiPi,
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| generated with Anthropic models at PsiPi's own expense, and are licensed
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| **CC BY-SA 4.0**.
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|
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| The **ChordStream column** is derived from the MIDI files in the same rows.
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|
|
| ## Citation and further reading
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|
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| The ChordStream format, its extraction procedure and its measured limitations are
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| described in the accompanying write-up:
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| <https://huggingface.co/blog/PsiPi/chordstream-a-key-relative-time-anchored-chord-tok>
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|
|
