Originally Posted by Nick @ Anthem
That makes me wonder how many broken movie soundtracks there are
Very few are broken if you ask me. But it’s not so black and white as being broken or not. Not all is perfection even for movies.
Regardless, the boom channel was...supposed to be limited to 120 Hz.
Yes. So if a content maker wanted to create an LFE signal that met this requirement, using his mixing console, what filter order and corner frequency would he use? A typical 120 Hz filter will not meet this requirement because filters have slopes. As shown in the graph, a 120 Hz 4th order LPF (green) is only down 6 dB at 120 Hz. The 80 Hz filter is down 14 dB (red) -- better, but still not optimal.
Have movie soundtrack production standards really deteriorated to the joke that run of the mill music "engineering" became a long time ago? Sure there will always be something falling through cracks for whatever bad reason but how widespread is this? How hard is it to follow the movie surround sound production guidelines?
It depends on how the guidelines are interpreted. Here’s what the DD Encoding manual says:
The LFE Lowpass Filter parameter can be used to activate a 120 Hz low-pass filter applied to the LFE input channel. If the digital signal fed to the LFE input does not contain information above 120 Hz, this filter can be disabled.
The 8th-order 120 Hz elliptic (blue) certainly meets the requirement, but it is not intended to shape the audible spectrum, just to limit the extent to avoid aliasing and minimize bitrate. The "sound" of a brickwall filter is rather odd when it is in the audible passband. Ideally the LFE spectrum would be defined by the 80 Hz plus the elliptic.
If the studio is crossing over the sub at 80 Hz as you suspect (and I don't doubt), shouldn't everyone else?
Just to be clear, I was describing an LFE sub limited to 80 Hz. No crossovers. And yes. Ideally LFE should be filtered
at 80 Hz (and limited to 120 Hz), and the filter should be in the signal path being recorded, not in the monitor path.
Let me mention something else. The heyday of theatrical 5.1 and LFE really came with the advent of digital film sound, even though they originated with 70mm mag. Not many people realize it but DTS theatrical was a 5.0 signal path, with the LFE spliced into the surrounds. The cinema processor split the signals with 80 Hz filters: HP to the surrounds; LP to the subs. This forced movies to be monitored with LFE limited to 80 Hz so that the results would be consistent on the stages and in theaters with any of the digital formats. But even then the recorded LFE tracks were not uniformly confined to 80 Hz. This became rather apparent at home when comparing a DTS track to the same soundtrack on DD (rare as that was). The "deeper, tighter" DTS bass was easily replicated by filtering the DD LFE to 80 Hz. Should we hear what the mixers heard and what was played in the cinemas, or what was recorded in the track?
So as you can see there are differing practices involved, and the results vary as a result. Depending on who you ask, the question of whether it is correct to reproduce everything delivered in the LFE channel has different answers. But unless the answer is "yes, always" then some sort of LFE filter option is needed, even when when ARC is running.