Digital inputs that are not routed (i.e., are not connected to "inputs block" or AES outputs) will not affect the status of the EXT LED.

See The Comprehensive Input / Output Scheme on page 49.

Using an External Clock

When you use the "external" clock, the system sampling rate will be that of the external digital signal. The system sampling rate is the sampling rate used for all internal processing. The external clock’s sampling rate is derived from the digital inputs shown in the table below, or the word clock input. Unless sample rate conversion is on ("bottom" clock menu page in the SETUP area), you must ensure that AES/EBU inputs 3/4 (and 5- 8 on H8000), if used, are synchronized to the selected clock input. Since no sample rate conversion exists for the ADAT, AES 11-17 and FireWire inputs, you must ensure that they are either synchronized to the source of the external clock or are in fact the source of the clock.

See Miscellaneous Setup Options on page 138 to enable/disable Word Clock input.

See the H8000FW FireWire Interface section for more information on using the H8000FW with a PC or Mac.

Note: If the clock source is "unlocked" (not connected to a valid digital source), the external clock will be invalid. If the external clock is invalid, the system sampling rate will revert to the last valid internal clock rate.

Selecting the External Clock

The external clock is derived from the inputs shown in the table above, or the word clock input. It cannot be derived from any other digital inputs.

To set the system sampling rate to the external clock’s rate, set the Source parameter on the clock menu page in the SETUP area to one of the available options. (S/P DIF and OPTO will be available if selected under the [inputs] menu page).

The system sampling rate will now be the sampling rate of the selected external input.

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Alternatively, you can use Word Clock as a clock source. H8000/H8000A users must enable Word Clock before use, on the misc or clock menu pages respectively in the SETUP area.

Word Clock is always enabled on H8000FW, so it need not be enabled.

Then you can select WORDCLK as the external clock source on the clock menu page in the

SETUP area.

The rate of the external clock is displayed in the "Sample Rate: xxxxx" field. In the example shown two screens above, the actual frequency of the external signal is 48001Hz. External sampling rates can range from ~30kHz to 53kHz and 61kHz to ~99kHz.

See Sampling Rates on page 79 for information on speed modes.

See Word Clock Termination on page 88 for information on connecting Word Clock.

Selecting a clock source when connected to a computer (H8000FW only)

When an H8000FW is connected to a computer by FireWire, the clock source is set by the computer and the source control on the H8000FW is disabled. This is because it is important that the computer and the H8000FW remain synchronized.

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The Status of the Digital I/Os When Using the External Clock

The right-hand side of the clock menu page in the SETUP area displays the status of the digital inputs.

DIN 1/2 reflects the status of the digital inputs 1/2 (either the AES/EBU 1/2 input jack or the S/P DIF 1/2 input jack depending on IN 1/2 on the [inputs] menu page).

DIN 3/4 (and DIN 5-8 on the H8000) reflect the status of the remaining XLR AES/EBU inputs. If you intend to use these inputs, you must ensure that these inputs are synchronized to the external clock source, as shown by the OK indication on the right hand side. ADAT reflects the status of the ADAT inputs - you must ensure that the ADAT inputs are synchronized to the external clock source if they are being used. Typically, when using ADAT, your clock source will be either the ADAT signal itself, or a WORDCLOCK to which the ADAT is also synchronized.

The H8000FW also shows the status of AES11-18 and FIREWIRE 1-2 – the same synchronization requirements apply.

The status of the digital inputs will be:

slipping DIN 1/2 will read slipping if its sampling frequency changes suddenly or if it is unstable. This means that signals are present, but they are not synchronized with the external clock input. The greater the discrepancy between the external clock and the input, the greater will be the distortion. Slight slipping can be acceptable in non-critical applications, as the occasional clicks it produces are often masked by the source material. Better, however, to enable sample rate conversion where available and be rid of it.

OK the input is synchronized to the external clock source and everything is cool.

unlocked no signals are present, the signals’ sampling rate is changing suddenly, the signals’ sampling rate is very unstable, or something is dreadfully wrong with the signals.

The sampling rate at the digital outputs will be the same as the sampling rate of the external clock (which is the system sampling rate in external ). A/D and D/A conversion will be done at this rate.

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Understanding the "System Sampling Rate and External Sync Indicator" When Using the External Clock

The "System Sampling Rate and External Sync Indicator" is the box of five LEDs immediately to the left of the display. The top four LEDs indicate the status of the system sampling rate:

Solidly lit: When one of the top four LEDs is solidly lit, the system sampling rate is exact (+/- 0.05%) (the LED corresponding to the system sampling rate will illuminate ).

Blinking: When one of the top four LEDs is blinking, the system sampling rate is between one of the fixed rates (the LED corresponding to the nearest sampling rate will blink ).

Use the "Sample Rate" field on the clock menu page in the SETUP area to see the actual sampling rate (e.g., 48001Hz in the example to the right).

The bottom LED, EXT, has two possible states:

Solidly lit: When the bottom LED is solidly lit, all of the routed digital inputs are locked and all is right with the world.

Blinking regularly: When the bottom LED is blinking regularly, at least one (and maybe more than one) routed digital input is unlocked (i.e., not synchronized to the external clock). Use the right-hand side of the clock menu page in the SETUP area to see which digital inputs are in fact unlocked.

To check which digital inputs are routed, consult the routing menu pages discussed in Routing Configuration

on page 53.

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