After the full review of the Tears for Fears album “The Hunting“, here is a focus on the comparison of the two Dolby Atmos versions: the streaming version proposed in Dolby Digital Plus and the Blu-ray version proposed in Dolby Digital TrueHD.
In addition to the technical analysis, I’ve included some extracts so that you can hear the differences between the two formats.
Dolby Digital Plus is a lossy compression format with a bit rate of 768 Kbits/s (for streaming), unlike Dolby Digital True HD, which is a lossless format offering variable bit rates of up to 10,000 Kbits/s. In the case of the album “The Hurting”, the maximum bitrate in Dolby Digital TrueHD is around 6500 Kbits/s. For both formats, the maximum number of channels available simultaneously is 16.
To get an idea of how compression works, the graph below shows the throughput of 16 uncompressed channels in 24 bits and 48 kHz, and the throughput of 16 uncompressed channels in 16 bits and 48 kHz.
Compared with the throughputs of Dolby Digital TrueHD at 6500 Kbits/s and Dolby Digital Plus at 768 Kbits/s, we can clearly see the efficiency of the encoders, particularly on Dolby Digital Plus.
By way of comparison, the graph also shows CD and Flac-encoded CD bitrates, with a 60% reduction in efficiency.
To achieve such a low bit rate, Dolby Digital Plus needs to optimize a number of parameters, starting with bandwidth. Despite using a sampling frequency of 48 kHz, giving a maximum signal frequency of 24 kHz (white arrow), Dolby Digital Plus limits itself to 20 kHz (yellow arrow), thus reducing bandwidth and associated information by 20%.
It also makes a further optimization in the bass channel, with a fairly high cutoff at 120 Hz (green arrow) .
The graphs below illustrate these limitations at 20 kHz and 120 Hz for the bass channel.
The graphs below show that Dolby Digital TrueHD offers the maximum bandwidth, i.e. 24 kHz (yellow arrow). With Dolby Digital TrueHD, the bass channel can be filtered or unfiltered (green arrow), depending on the choices made.
If we make a similarity with stereo formats, we could say that Dolby Digital Plus corresponds to Mp3 or AAC, while Dolby Digital TrueHD corresponds to FLAC.
What is it really like to listen to this difference in bit rate will impact the amount of information that can be produced, and therefore the finesse in terms of sound restitution, but also in terms of spatial precision. The extracts below compare the two Dolby Atmos formats downmixed to 7.1.
To get a better idea of the impact of compression and the signal degradation that can occur, particularly on the top channels, here’s a comparison of the left and right front top channels of the Dolby digital plus version and the Dolby digital TrueHD version.
In the Dolby Digital Plus version, some artifacts are audible, diminishing reproduction quality.
Listening to the top-front channels individually but well in front is qualitatively different, but when listening to all the channels together, this difference is less audible and is transformed mainly into a lack of precision in rendering and spatialization.
It should also be noted that these artifacts are not systematically present in all tracks and depend on the difference between the sound levels of the various channels.
Dolby Digital TrueHD is therefore the best format for Dolby Atmos rendering, but unfortunately it’s not widely used in music and is only available on Blu-Ray discs, or as a download in MKV format for certain albums, as was the case with album “A Shade Of Blue” from Tsuyoshi Yamamoto Trio.
We can only hope that this format will become widespread, whether on Blu-ray media for download or on streaming services, which have been able to offer high-resolution formats and which could offer Dolby Digital True HD for maximum quality in Atmos.
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Quite weird that in the TrueHD version, the LFE (is it still named this way?) channel is not filtered at all. When listening to it alone, it seems that the bass drum and bass guitar track are put in this channel unmodified.
I guess that’s the situation where a setting like « LPF for LFE » (default at 120 Hz) on my Denon AVR kicks in. But what will it do? Move the high frequency content to L+R? Or to all channels? Or discard it?
In E-AC-3 only 5.1 channels are encoded. The rest are parametrically derived from these, similar to parametric stereo of old, by cutting out a frequency range, which can change instant to instant. You can hear this better if only some dynamic drums are panned to the additional channels. The parameters occupy around 70 kbit/s. This leaves a compatible surround stream with about 700 kbit/s, which is quite good and better than DVD.
SACD and DVD-A could also have unfiltered LFE. On downmixing the LFE is often discarded. Lowpass will likely do what it says and completely remove higher frequencies. The instruments in the bass channel likely are duplicated in other channels such as the center.
Thanks for the comment, but it’s a bit more complicated. In fact, in Dolby Digital Plus (for audio streaming) with a bit rate of 768 kbits/s, there’s a basic 5.1 mix, then there’s Joint Object Coding, which adds the information needed to reproduce up to 16 channels (including LFE).
This information is reconstituted with the JOC information and extracted from the 5.1 version.
Dolby Digital Plus is 2 times more efficient in compression than Dolby Digital, which also explains the difference in quality with Dolby Digital.
Dolby Digital Plus requires LFE filtering, unlike DSD64 for SACD or Dolby TrueHD for DVD-A or Bluray Audio.
The LFE is generally not included in the donwmix, as it contains only the extra bass, the other channels containing the normal bass.
LFE should therefore be used very little or not at all in music.
Thanks for the comment, but it’s a bit more complicated. In fact, in Dolby Digital Plus (for audio streaming) with a bit rate of 768 kbits/s, there’s a basic 5.1 mix, then there’s Joint Object Coding, which adds the information needed to reproduce up to 16 channels (including LFE).
This information is reconstituted with the JOC information and extracted from the 5.1 version.
Dolby Digital Plus is 2 times more efficient in compression than Dolby Digital, which also explains the difference in quality with Dolby Digital.
Dolby Digital Plus requires LFE filtering, unlike DSD64 for SACD or Dolby TrueHD for DVD-A or Bluray Audio.
The LFE is generally not included in the donwmix, as it contains only the extra bass, the other channels containing the normal bass.
LFE should therefore be used very little or not at all in music.