SPL Measurements of LDs and DVDs
Bjoern Roy (last updated September 11th 2002)

There will always be discussions which DVD or LD has 'the most bass', or which version (LD or DVD, DD or DTS) of a film 'sounds best'. An analysis of the latter is impossible, because its of totally subjective nature. What we can do, though, is measure the SPL (sound pressure level) of the individual channels in a DVD or LD soundtrack to make a quantitative instead of a qualitative judgement.

So, while we can't really measure whether track A has a 'wider soundstage', 'tighter bass' or 'more air between the instruments' than track B, we can show for example that one track has more bass in a certain freqency range than another. Or that one track still has the highs boosted, like the original theatrical mix, probably requiring THX's CinemaEQ to tame them, while the other might already be mixed for a near-field home presentation, with the proper reduction of the highs.

It has to be noted that comparing two tracks doesn't really show which one is correct per se. If one track has more bass than the other, then there is the possibility that the weaker one is correct and the stronger one is boosted beyond the sound mixers original intend. But its also feasible that the stronger one is untouched, while the weaker one is toned down for home use, which is unacceptable from a audiophile perspective.

So the measurements and graphs can only be a tool to investigate the differences. Its up to the experience of the investigator and his proverbial educated guess to come to likely conclusions.

 

Titles in the Spotlight

 

Setup and Proceedings
Here is a brief description of the test setup:
  • DD/DTS bitstream from DVD/LD transport into my Lexicon MC-1 decoder
  • all channel levels to +-0db, since we don't want to measure in-room response, but rather the direct output of the decoder
  • preouts of MC-1 directly into PC sound card for spectrum analysis
  • disabled rear surrounds (only side surrounds) to get straight 5.1, not 7.1
  • no special Lexicon processing (Logic7 etc...), only pure DD and DTS modes
  • all speakers set to LARGE, so that each channel's output correlates with the encoded channels without any bass-management; this yields readouts for the L, C, R, SL, SR and Lfe channels
  • only set all speakers to SMALL to get bass readouts (lfe+rerouted bass) on Dpl, DD or DTS tracks
  • same overall volume used for all tracks, so that 0db on the vertical scale of the graphs indicates 115db SPL, which is only achievable by the Lfe channel; all non-lfe channels max out at 105db, thus -10db on the scale
  • compensate for Dialog Normalization, so that DD and DTS tracks play at even overall volume levels
  • paying a lot of attention that the exact same scene is used for when comparing tracks; easily achievable by starting and stopping at quiet passages
  • Lfe data above 120hz is irrelevant!
  • horizontal scale usually from 5hz up to 20kHz, see at bottom of graphs
  • legend of colors is always at the right sideof the graphs; not all colors necessarily used in every graph
  • the headings left channel and right channel at the top of the two graph windows are meaningless; the channel that is actually measured is indicated in the color legend or description only

 

 

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