[Discussion] About training models myself

Below is the content from [Here](https://github.com/deezer/spleeter/wiki/2.-Getting-started#train-model) and I've read it.

### Train model
For training your own model, you need:

- A dataset of separated files such as [musDB](https://sigsep.github.io/datasets/musdb.html).
- Dataset must be described in CSV files : [one for training](https://github.com/deezer/spleeter/blob/master/configs/musdb_train.csv) and one for [validation](https://github.com/deezer/spleeter/blob/master/configs/musdb_validation.csv)) which are used for generating training data.
- A JSON configuration file such as [this one](https://github.com/deezer/spleeter/blob/master/configs/musdb_config.json) that gathers all parameters needed for training and paths to CSV file.
Once your train configuration is setup, you can run model training as following :

```
spleeter train -p configs/musdb_config.json -d
```

From the command above, I notice that I need to provide:
- A `musdb_config.json` file;
- A `musdb`

---

### Question 1
A `musdb_config.json` file look like below, copy from [here](https://github.com/deezer/spleeter/blob/master/configs/musdb_config.json)
```
{
"train_csv": "configs/musdb_train.csv",
"validation_csv": "configs/musdb_validation.csv",
"model_dir": "musdb_model",
"mix_name": "mix",
"instrument_list": ["vocals", "drums", "bass", "other"],
"sample_rate":44100,
"frame_length":4096,
"frame_step":1024,
"T":512,
"F":1024,
"n_channels":2,
"n_chunks_per_song":40,
"separation_exponent":2,
"mask_extension":"zeros",
"learning_rate": 1e-4,
"batch_size":4,
"training_cache":"cache/training",
"validation_cache":"cache/validation",
"train_max_steps": 200000,
"throttle_secs":1800,
"random_seed":3,
"save_checkpoints_steps":1000,
"save_summary_steps":5,
"model":{
"type":"unet.unet",
"params":{
"conv_activation":"ELU",
"deconv_activation":"ELU"
}
}
}
```

But where can I get the full documentation of it? For example, what does `T` and `F` mean? for `instrument_list`, can I only use `["vocals", "other"]`? where can I get the full documentation of all these config options?

---

### Question 2
I've downloaded the musdb from [musdb18.zip](https://zenodo.org/record/1117372#.YjBJb69Bw0o) and extract the zip file, I found that it is a folder containing 2 folders: `train` and `test`(see the screenshot below)

Inside the `train` and `test` folder, there are all mp4 files(use mp4 instead of mp3 or aac is because mp4 can contain more than one track in it)

I've listened the mp4 file in `train` folder and `test` folder in musdb18.zip, it seems they are no any difference, they are all songs.

So in my understanding, they are no difference. Assuming that I have 150 song file, can I choose 100 of them for training and the rest for validation?

---

### Question 3
I use `ffprobe` to check the `mp4` files mentioned above, I found it has many tracks, the first track is a mix of all audio tracks, the other audio tracks are separated tracks(vocals, drum, bass, .etc), and the last track is a video track, but in fact it has no video, it's a still png image.
```
ffprobe -hide_banner -i Young\ Griffo\ -\ Pennies.stem.mp4
[mov,mp4,m4a,3gp,3g2,mj2 @ 0x7f8bfc808200] stream 0, timescale not set
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'Young Griffo - Pennies.stem.mp4':
Metadata:
major_brand : isom
minor_version : 1
compatible_brands: isom
creation_time : 2017-12-16T17:34:20.000000Z
Duration: 00:04:37.80, start: 0.000000, bitrate: 1288 kb/s
Stream #0:0(und): Audio: aac (LC) (mp4a / 0x6134706D), 44100 Hz, stereo, fltp, 256 kb/s (default)
Metadata:
handler_name : SoundHandler
vendor_id : [0][0][0][0]
Stream #0:1(und): Audio: aac (LC) (mp4a / 0x6134706D), 44100 Hz, stereo, fltp, 256 kb/s
Metadata:
handler_name : SoundHandler
vendor_id : [0][0][0][0]
Stream #0:2(und): Audio: aac (LC) (mp4a / 0x6134706D), 44100 Hz, stereo, fltp, 256 kb/s
Metadata:
handler_name : SoundHandler
vendor_id : [0][0][0][0]
Stream #0:3(und): Audio: aac (LC) (mp4a / 0x6134706D), 44100 Hz, stereo, fltp, 256 kb/s
Metadata:
handler_name : SoundHandler
vendor_id : [0][0][0][0]
Stream #0:4(und): Audio: aac (LC) (mp4a / 0x6134706D), 44100 Hz, stereo, fltp, 256 kb/s
Metadata:
handler_name : SoundHandler
vendor_id : [0][0][0][0]
Stream #0:5: Video: png, rgba(pc), 512x512 [SAR 20157:20157 DAR 1:1], 90k tbr, 90k tbn, 90k tbc (attached pic)
```

Now I have vocals.m4a and bg-musics.m4a, how can I merge a vocal and it's corresponding bg-music and a album cover image to a mp4 file by using ffmpeg?

---

### Question 4
From question1 we can know that we also need 2 csv file: `musdb_train.csv` and `musdb_validation.csv`.

I notice that the [musdb_train.csv](https://github.com/deezer/spleeter/blob/master/configs/musdb_train.csv) file has 6 columns:
```
mix_path | vocals_path | drums_path | bass_path | other_path | duration
```

If I only needs 2stems, it that I only need to provide these 4 columns in this csv file?
```
mix_path | vocals_path | other_path | duration
```