Conversion Confusion!

siog

New member
Hi all,

Can someone shed some light on this feature in the dbx386 preamp -

"The 376 uses the proprietary Dbx Type IV A-D conversion system, which is designed to extend the headroom for any given digital bit-depth by introducing a non-linear region that affects only high-level signals. In this case, the top 4dB of the dynamic range is made logarithmic rather than linear — so that the further signals push into this region, the more they are squashed. In fact, the log law means that you can never get to the point of digital clipping and, because the logarithmic law mimics tape and tube distortion, it can actually enhance the signal to make it sound more 'analogue'. A further benefit is that, by avoiding clipping, high frequency detail is preserved, albeit in a slightly distorted form, whereas clipping would obliterate it altogether. As Type IV is unclippable, a separate limiter is also unnecessary."

If a preamp doesn't have a feature such as this, what does it mean in simple English?

Cheers
 

Bobbsy

Boring Old Git
Okay, that's one of the best examples of technical mumbo jumbo written. It's designed to sound impressive.

What it means is that there's a soft limiter with a roughly 4:1 ratio for levels above about -4dB(FS). Supposedly this emulates an analogue chain that goes into soft clipping (an effect that some like) before it starts to sound horrible.

What does it mean if a preamp doesn't have this? It means you have to get your gain staging right so you don't clip in the first place--or use a separate limiter in the chain.

Don't get me wrong, the dbx is a pretty good pre amp and the "analogue simulation" while not sounding exactly analogue can be nice on some things.

However, I really, really don't like manufacturers who try to big themselves up with almost meaningless technical buzz words.
 

Armistice

Son of Yoda
This would be a "nice to have" feature if it was able to be turned off. If it does it automatically, not sure I'd want it.
 

Bobbsy

Boring Old Git
Good point. I'd want to be in control.

...though it sounds like it shouldn't have much effect unless you push your levels above -4dB(FS) anyway (which is normally a bad idea (unless you deliberately want the limiter to kick in of course).
 

siog

New member
I had a reply from Harman about the dbx feature - the Type IV A/D conversion can not be switched off or bypassed when sending a digital signal.

One more question - would it make more sense to buy a preamp without an A/D converter, e.g: the Art Pro MPA II, and a separate A/D converter such as Aphex D 500 Duo 500-series Chassis ?

Is it usual to use a preamp with an A/D converter or is there an argument for having the preamps and converter housed in separate units?

Cheers :)
 

Armistice

Son of Yoda
Most of us use audio interfaces with the preamp and converter in the same unit. You can usually bypass the preamps using a line signal from something else if you wish to.

I'd avoid getting anywhere near the last 4dB during tracking anyway. You should be averaging -12 to -18 - that's the accepted wisdom down here.

If I were you I'd stop worrying about separate units, and work out the best audio interface you can afford with the number of channels you need if you're relatively new to this. You can worry about high end standalone preamps and converters later when you've made your first million... there are almost always other factors that you need to spend money on first before getting into that type of kit in your typical home recording set up...

Of course if you've got all that covered already, know exactly what you're doing and have the cash, knock yourself out...
 
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