In DAB and DAB+, we call the equipment before the DAB transmitter the “Headend.” The equipment consists of the DAB+Audio-encoder, the DAB+ Pad-encoder, and the DAB+MUX.
This equipment can be distributed (over several providers) and connected over STI links. When all equipment is available on only one site, it can be connected over an Ethernet LAN.
The whole of the “Headend” equipment allows the MUX to generate the ETI signal for the transmitter.
The transmitters in an SFN network are always on multiple transmitter sites, and the ETI signals must be transported over communication lines from the DAB-MUX to the different DAB transmitters. So, the ETI interface can come in different flavours.
Our glossary refers to ETI NA, ETI, and EDI. These interfaces can be confusing, mainly because they use the same transmitter RJ45 connector.
Be careful:
For audio quality reasons, we advise placing at least a part of the “Headend” the DAB+ encoder (including the PAD encoder) near your broadcasting studio and connecting it with digital AES/EBU or AES67 audio interfaces.
- Do not use lossy formats as an audio source.
- Use at least 16-bit bit-resolution for source material.
- Use a 24-bit digital mixer and 24-bit DSPs in your audio chain.
- Know that DAB+ operates on 48kHz sample rate (some MUXes uses 32kHz)
- Know that DAB+ uses HE-AAC with a 120/960 sample window (instead of the usual HE-AAC 128/1024 sample window)
- Therefore, do not compare a DAB+ Audio-encoder with the standard HE-AAC encoder, You cannot replace one with the other
- Provide your lossless broadcasting signal directly to a local DAB+ encoder.
- Even if you use good quality compression, leave at least nine (9) dBfs headroom at the input of your Audio-encoder (to avoid any audio distortion due to hard clipping).
- Use a DAB+ encoder with digital AES/EBU or AES67 inputs in your studio and connect it with a digital connection. It will avoid multiple or cascaded encoding.
With the above recommendations, you can certify an optimal audio quality signal for your DAB+ transmission. Having the DAB+ Audio-encoder in the studio guarantees that your lossless quality signal is directly converted to a lossy (HE-AAC) high-quality signal fed to the DAB+ MUX and the DAB+ Transmitter.
In our experience, not taking the above advice seriously will degrade DAB+ audio quality in DAB+ radio projects. You can still obtain an acceptable quality by ignoring one or two recommendations. Still, by violating too many of these recommendations, your audio will not sound good—probably worse than your analogue FM transmission.