DP-006 schematics bug?

axd1967

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DR-8, DP-006
Hello,

I have a question on the Tascam DP-006 schematics (see UM, page 101).

The signal path seems to be:
gain -> A/D -> LEVEL -> OL -> ...

but the OL can light up even when LEVEL is all the way to the left (down), so I wonder if the sequence wouldn't instead be

gain -> OL -> LEVEL -> A/D,

or,

gain -> A/D -> OL -> LEVEL ?

could this be a mistake in the diagram, or in my head?

thx

block.jpg
 
Last edited:
The signal path seems to be:
gain -> A/D -> LEVEL -> OL -> ...

but the OL can light up even when LEVEL is all the way to the left (down)
This seems indeed strange. Usually one wants to know whether overload is taking place in the analog domain. If it CAN overload with LEVEL down this means the OL measurement must be taking place before the A/D conversion.
 
I first thought that OL LEDs were reflected in the A/B OL LCD indicators, but it can happen that OL is lit while A/B LCD overload indicators are not.

The manual indeed states (p53) that OL can be lit while LEVEL is MIN.

I think OL logic is a bit more complicated, because there is

- OL LED per input channel (A/B)
- OL LCD bar per channel (A/B) (not necessarily linked with LED OL!)
- OL LCD bar per track (1-4)

Where exactly LEVEL intervenes is not very clear.

The DP-006 seems to store >0db data that Audacity will clip:
m2.jpg


channel B of the recording was analysed.

some terms:

- A,B: channels (left, right)
- LEVEL: channel potentiometer
- A/B OL: LED that indicates input overload. initially this seems to correspond to the LCD OL indication above the A/B bars (but that is not true)
- track OL: horizontal bar in LCD above the track level, indicating some overload has occurred in the track

diagram events:

In following events, a synth was connected to A/B and its volume was turned down starting from its max level. A channel LEVEL was MIN, B LEVEL was MAX, DP-006 gain was high (to more easily trigger OL).

1- A+B OL: input overload, decaying RMS (input level is being decreased) shows DP-006 recorded volumes above 0db, Audacity seems to cut away that data.

Also, channel A LEVEL is zero(!), yet A OL is lit.

2- B OL lit

3- still B OL, but track OL off

4- B OL off

At this point, DP-006 amplifier is switched from high to med to low in order to get an idea of the different amplifier gains:

5- high gain: around -2.5db (as measured from the vertical scale, I have no idea how precise this is)

6- medium gain: around -8.5db

7- low gain: around -14.5 db. Note that here, although you can't see it in the diagram, input and track levels are touching the LCD "-12db" line. It appears to be difficult to modulate a signal's volume to hit exactly the "-12db" line, because of the low resolution of the LCD.

So the amplifier can boost 6db per stage (low->med->high)
In each of these gain settings, Audacity playback meter shows +0.5db from these values.

A bit further (around 3:30, not shown in the diagram), I put LEVEL at 50% and turn down source volume until track levels are around "-12db" line. and indeed, Audacity also shows (around) -12db.

Interestingly, input OL does not correspond to A/B OL indication.

----

So I think the diagrams are of low quality. For example, the DP-004 (the previous model) is even worse, while the DP-008 seems more accurate.

dp004.jpg
dp008.jpg
 

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