Sonifex HY02 DIGITAL, User Handbook Manual

 Circuit Description & Controls         Page 2-5                 

HY02 Digital TBU User's Guide 

 
Maximum output level to line (soft limited) : +6dBM 
Tx LED sensitivity :  

-7dB on, -10dB off 

Output noise to line : 

-66dB 

2.2.5.4  Receive Circuit 

Receive set to low:   

 
Gain 

: 

    +9dB 

Maximum output : 

+8dB (soft limited) 

Distortion 

: 

   0.1% 

before 

limiting 

Maximum input level :    

20dBm 

Noise 

: 

    -66dB 

(-77dB 

off 

line) 

Rx LED sensitivity : 

-14dB on, -17dB off at output 

Frequency response :  

-3dB 250HZ, flat to 3.5 kHz. 

 
The cancellation uses FIR filters with a normalised signal dependent adaption 

algorithm. The adaption is disabled during double-talk and when the signals are too low in level.  The 
adaption coefficients are held digitally and are not subject to drift when no signals are present.  
Typically 40dB of rejection is achieved with real signals although 60dB rejection can be measured 
using test signals into dummy lines. Send and receive levels are set up by on-board links. 

Although the board works in full duplex mode, links can program signal dependent 

half duplex operation; this enhances the overall sidetone rejection/stability with little degradation to the 
apparent full duplex operation. 

2.2.5.5  Hardware Control 

 
The DSP is used to control all facets of the operation of the board. On power up, the 

digital devices are checked for correct operation and a status led flashes if errors are found. The DSP 
reads the onboard links and sets the operational mode signal levels accordingly. The DSP also controls 
the divert status of the board via the codec auxiliary data I/O channel. The signals are software 
‘debounced’ for stable operation. 

The send and receive LEDS are processed via the DSP. These have hysterisis built in 

to minimise ‘chattering’. The divert relay and two/four wire mode control is driven via the DSP. 

The signal is checked for overload, and digitally limited if needed. The ‘Tx LED’ is 

turned on if the outgoing signal is above -14dB and the Tx signal is gain reduced if found to be below -
24dB. The Tx level is adjusted before being sent to the codec send port. The signal is processed in 
16/32 bits throughout, to maintain good audio performance. The receive signal from the codec is 
processed via the DSP before being presented to the output. The receive signal is checked for excess 
level and the input analogue signal is limited if the signal is overloading. The signal is then high-pass 
filtered and, if enabled, network cancellation is implemented. 

The signal is checked for overload and digitally limited if needed and the receive is 

disabled if the board is ‘off line’. The ‘Rx LED’ is turned on if the signal is above -14dB and signal is 
gain reduced ( i.e. noise reduction is introduced ) if it is below -24dB. The signal is also gain reduced if 
levelling is desired, and the send signal is dominant. Receive level is adjustable from 9 to 21dB.  

2.2.5.6  Digital Cancellation Control 

 
Fixed sidetone cancellation occurs in two wire mode prior to DSP processing. Full 

adaptive network cancellation is achieved by DSP means. Echo cancellation is 28ms. 

2.2.6.  Ring Detect Circuits 

 
The ring detector circuit is optional. It is permanently coupled to the telephone line at 

the handset relay contacts and is isolated from the line by the optocoupler Q13. 

Ringing tones are detected by the circuitry formed by Q12, Q11, Q10 and Q9. Ring 

sensitivity is adjusted by RV3. The output of the ring detector is coupled to the divert switch latch by 
Q7 and the jumper connection JP3.