Re: OT Discrete Surround Sound Speakers

["Graham Howe" <graham@xxxxxxx>]

--- In ukha_d@y..., "Mark Harrison" <Mark.Harrison@e...>
wrote:
> Paul,
> 
> I'd suggest that you start by finding a different shop :-)
> 
> There are two different things going on here about the "would not
know what sounds were supposed...", neither of which has anything to
do with whether the s/w is active or not.
> 
> Firstly, the old-fashioned way of sending signals to a subwoofer 
was to feed the front channels into it _as_well_ as to the front pair 
as speaker signals. The s/w would then have a low-pass filter (a 
resistor/capacitor network) that filtered out high frequencies within 
the speaker electronics, and then passed only the low frequencies 
remaining onto the s/w cone.
> 
> The newer way of doing this is to use a processor to do the 
filtering BEFORE the signal gets sent to the speaker, and therefore 
has a dedicated connection on the processor for a subwoofer. 
Depending on the source material, it may do this simply by applying a 
low-pass filter, or by decoding digitally what was intended for a 
bass channel. Only material encoded with one of the most modern 
formats (Dolby Digital (AC-3), DTS) etc. have this channel. The older 
formats (Stereo, Dolby Pro-Logic) don't.
> 
> Neither of these require the s/w to be active.
> 
> All active means is "it has a built-in amp". In the second
case, 
this means that you take the "s/w out" from your processor, not
as a 
speaker cable connection, but as a "line-level" connection,
probably 
on a 35mm phono socket. (In the former it's slightly more complex, 
but basically re-amplifies an already-amplified signal, and probably 
has a more sophisticated cross-over that filters the LOW frequencies 
out of the signal that goes onto the main pair as well.) (Yes - I 
know I'm oversimplifying here, Phil!)
> 
> I would be very dubious of any hi-fi shop that confused 
active/passive with filtering/dedicated channels, and would only look 
to buy hi-fi from a dealer who understood it better than me - not 
worse!
> 
> Regards,
> 
> Mark
> 
Whilst I agree with everything Mark has said (particularly about 
finding a better HiFi dealer), I think it is worth emphasising that 
most modern processors in a AC3/DTS environment are going to have the 
subwoofer output coming out at line level. In other words the AV 
amp/receiver is likely to only have 5 channels of actual 
amplification even though the subwoofer signal is extracted as a 
separate channel from the source.

What this means is that if you have a passive (no internal 
amplification) sub woofer and want to use it in a 5.1 environment, 
then you are going to want to add a mono power amp between the AV amp 
and the sub.

The more usual approach is (as Mark described) to use a powered sub, 
then everything works as expected.

Giving the HiFi shop that you visited the benefit of the doubt (not 
that they deserve it), they could be saying that as the 'bass' part 
of the the Bose system (not sure if this was an actual sub or not) 
was passive, then it would not be appropriate for use in a typical 
5.1 environment *without* the addition of a mono power amp.

HTH

Graham

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