I’ve noticed that many stations still use MP3 at bitrates of 128, 64 and sometimes 32 kbps in 2024. Those formats are obsolete since a long time and and have the disadvantage of generating a lot of unpleasant artifacts in the high frequency range.
AAC is now 100% compatible on every devices since many years and offers more quality for the same bandwidth used.
If you want to keep the 3 bandwidth configuration for those users with low bandwidth access, I would suggest the following:
using FFMPEG (non-free) you get access to libfdk-aac, this library allows for aac_he / aac_he_v2 and faster encoding that the built in AAC encoder.
For those settings suggestions, keep in mind that, at low bitrates, fidelity is more important than stereophony.
low bitrate: 27 kbps aac_he, mono (works over dial-up and fidelity is acceptable ) ((Reason I choose 27 instead of… 28 kbps is that the codec raise the non-sbr bandwidth frequency cutoff every given bitrate and 27 kbps give a few more bits to the non-sbr portion than 24 kbps.))
med bitrate 64 kbps aac_he_v2 Stereo ( compromise for those overseas that wish to save bandwidth )
high bitrate 192 kbps aac-lc Stereo ( almost full CD quality ) (( That is what CBC now use ))
Keep in mind that it is not required anymore with AAC to use lower sampling rate such as 22050… 44.1 /48 works fine with all the configurations stated above.
Also, keep in mind that by default, libfdk-aac seems to have a cutoff at 17kHz @ 192 kbps which is perfectly fine. If you want to enforce it, you can use the flag “-cutoff 17000” . It’s not necessary to raise it above that frequency, neither you and I don’t really hear above that point. Allowing the codec to keep frequencies above that point will only allow less bandwidth to be used for the frequencies we still can hear.
Here are some sample files encoded with the three configurations stated above.
But what about OPUS?
It’s true that OPUS pack a lot of quality in some average bandwidth but it lacks the SBR feature HE_AAC benefits from at very low bitrate.
If you only have 32 kbps available, you’re still better off using HE_AAC than OPUS for a better