Which audio format to use on the STL?
As we wrote here, the DAB(+) audio encoder sends the encoded DAB(+) audio from the radio programme to the DAB(+) MUX. We describe now how to send audio to the DAB(+) audio encoder.
Let us warn once more time that the quality of the source material should be optimal. If quality is important to you and you want to compare your audio quality with the pro commercial DAB(+) radios. In that case, you probably should not use less than lossless files (WAV, FLAC, ALAC) as source material.
1. Via Icecast or SHOUTcast
It is by far the easiest and cheapest way, but it will often have some drawbacks in audio quality.
When the radio broadcaster’s web stream is of excellent quality, one can consider using this web stream as the audio STL. Because a digital conversion takes place in the DAB(+) encoder, it is important to use the highest possible bitrate: 96 kbps HE-AAC, 256 LC-AAC or at least 320 kbps mp3 as an absolute minimum.
Setting up a dedicated web stream for your DAB(+) STL is even better. If a high-bandwidth connection is available, you can even stream in FLAC. As FLAC is a so-called “lossless” audio compression format, FLAC will therefore give the best results when chosen as STL for a web stream. The continuous available throughput or bandwidth of the internet or the network connection will be the determining factor. If audio interruptions frequently occur in the audio stream, the available bandwidth over time is probably insufficient, or the buffer length at the receiver side is too small.
Figure 1 – Easiest way with possible audio artifacts: DAB+ encoder at provider site
2. Putting the DAB(+) audio encoder in the studio
When the DAB(+) audio encoder is placed in the studio (or final direction), the audio output of the audio processor can be linked directly to the LINUX PC. ODR mmbtools supports all common audio connection formats (analogue, AES/EBU, livewire(AES 67)). Delivering the correct DAB+ HE-AAC format by a DAB+ encoder behind the radio console will ensure the best end-to-end audio quality.
Figure 2 – Best audio quality: DAB+ encoder at studio site
The DAB(+) audio encoder will send the “DAB AAC+” format via “zero MQ” or “TCP/IP” immediately to the DAB+ MUX. Any further transcoding will be unnecessary. It can be done via the internet or a dedicated MPLS VPN connection (higher availability).
3. Audio over fibre optic, landlines or radio link
Sending digital audio (AES/EBU) is sent over a glass fibre network or landlines to the DAB(+) audio encoder is a possibility, but is quite unaffordable for small-scale projects.
It is not always possible to set up radio links everywhere. A professional radio link almost always requires a licence with associated costs. The choice to use a radio link should be on a case-by-case basis.
Remember: good sound quality always starts with the quality of the audio files.