ffmpeg bitrate control

[small tapir]

is there any way i could get finer control over bitrate when encoding a file? (would this be different depending on the encoder im using?) like decreasing or increasing the bitrate with some sort of math function or something? also how would i google things like this, when i try to search questions like this i get streaming bitrate settings back which kind of isnt what i want

[slate pawn]

You want exact control over bits per frame?

Probably not exactly

You MIGHT be able to do some fuckery with i-frames and lossless merging

Fascinating question though, I will look into this later

[small tapir]

thank you frost my goat

i am planning on using something like this for a musical piece for one of my classes, so if its possible to do something like this with aac or opus or similar that would be perfect for my usecase

[slate pawn]

I believe you could do this with webm at the very least

or somehow inject something into the encoder to make it target a different bitrate/quality as time passes

[slate pawn]

@small tapir IVE DONE IT

solution is to segment the file

then reeencode all separately

then change the bitrate to a custom equation

import os

inFile = "gun.flac"
# inFile = input("Input file: ")
nameFile = "out"
extension = 'flac'
# extension = input("Output file type (no .): ")
timeSplit = 1
# timeSplit = float(input("Split every n seconds: "))

os.system(f"ffmpeg -stats_period 0.05 -hide_banner -loglevel error -y -i {inFile} -c copy -flags +global_header -segment_time {timeSplit} -f segment {nameFile}%d.{extension}")

numOuts = int(input("Last output number?"))

# linear decrease in bitrate
startBitrate = 128

for i in range(numOuts+1):
    # command = f'ffmpeg -stats_period 0.05 -hide_banner -loglevel error -y -i {nameFile}{i}.{extension} -c:a aac -b:a {int(startBitrate-(startBitrate*i/numOuts))}k {nameFile}{i}new.aac'
    command = f'ffmpeg -stats_period 0.05 -hide_banner -loglevel error -y -i {nameFile}{i}.{extension} -c:a aac -b:a {int(startBitrate*(numOuts-i)/numOuts)}k {nameFile}{i}new.aac'
    print(command)
    os.system(command)
    os.remove(f"{nameFile}{i}.{extension}")


with open("output.txt", "a") as f:
    for i in range(numOuts+1):
        print(f"file '{nameFile}{i}new.aac'", file=f)

command = f'ffmpeg -y -stats_period 0.05 -hide_banner -loglevel error -f concat -i output.txt -c copy concatenated.aac'
os.system(command)

for i in range(numOuts+1):
    os.remove(f"{nameFile}{i}new.aac")
os.remove("output.txt")

for "last output?" enter the highest number of output.flac (or whatever your extension is) that it generates

also edit the top variables as needed

output

currently just linear decrease in bitrate (so not a linear decrease in subjective quality)

you could adjust this to any equation you'd like

startBitrate/(i+1) gives exponential decay

[small tapir]

THANK YOU FROST MY GOAT

[slate pawn]

its not fully working i lied

im working on it rn though

currently concatenation stops after like 1/3 of the file

i think the bitrate gets too low honestly

MEVERMIND

I was stupid

it was just fucking the duration up!

import os, math

inFile = "gun.flac"
# inFile = input("Input file: ")
nameFile = "out"
extension = 'flac'
# extension = input("Output file type (no .): ")
timeSplit = 6
# timeSplit = float(input("Split every n seconds: "))

os.system(f"ffmpeg -stats_period 0.05 -hide_banner -loglevel error -y -i {inFile} -c copy -flags +global_header -segment_time {timeSplit} -reset_timestamps 1 -f segment {nameFile}%d.{extension}")

numOuts = int(input("Last output number?"))

# linear decrease in bitrate
startBitrate = 256

for i in range(numOuts+1):
    # command = f'ffmpeg -stats_period 0.05 -hide_banner -loglevel error -y -i {nameFile}{i}.{extension} -c:a aac -b:a {int(startBitrate-(startBitrate*i/numOuts))}k {nameFile}{i}new.aac'
    command = f'ffmpeg -stats_period 0.05 -hide_banner -loglevel error -y -i {nameFile}{i}.{extension} -c:a aac -b:a {int(startBitrate/(i+1))}k -fflags +genpts {nameFile}{i}new.aac'
    print(command)
    os.system(command)
    
    os.remove(f"{nameFile}{i}.{extension}")

# issue is not fixed by remuxing, but is fixed by not dropping the bitrate so low
# const bitrate of 50k had no issues
# is definitely caused by low bitrate - how to preserve info?

with open("output.txt", "a") as f:
    for i in range(numOuts+1):
        print(f"file '{nameFile}{i}new.aac'", file=f)

command = f'ffmpeg -y -stats -stats_period 0.05 -hide_banner -f concat -i output.txt -c copy concatenated.aac'
os.system(command)

for i in range(numOuts+1):
    os.remove(f"{nameFile}{i}new.aac")
os.remove("output.txt")
input("done!")```

@small tapir fixed and working

currently does an exponential decrease in bitrate

maybe

it just divides bitrate by the segment number

first 6 seconds 256/1
next 256/2
next 256/3 etc

30 second example

[lone basalt]

what is ur usecase