You may wonder - what's that strange connector on the top of the Bullfrog? This is the VOICECARD slot. Voicecards expand functionality of the Bullfrog and make it even more versatile. Essentially, the voicecards replace patch cables - they connect the "modules" of the Bullfrog internally. But that's not all about them - some voicecards have built in mixers, signal generators and advanced electrical circuits to control the synthesiser, for example, sequencer that sends CV to control pitch and gate to start events on the synthesizer (Figure 40).

When inserting voicecards, make sure, the front panel of the voicecard faces the main interface of the Bullfrog. It is recommended that you turn off the Bullfrog when inserting a new voicecard.

As it's name suggests, the HIGHPASS VCF voicecard expands the sound treatment capabilities of the Bullfrog by bringing a highpass filter into the game. This allows for exploring highpass and bandpass filtering. The voice card features typical VCF controls - CUTOFF frequency, RESONANCE and CV LEVEL. The CV source switch allows for selecting between three CV sources - envelope generator 2, Sample & Hold LFO, which is a pulsewave generator in the Sample&Hold "module". The voice card features internal audio connections - the output from the MIXER is patched into the highpass VCF, the output of the highpass VCF goes into the input of the onboard lowpass VCF which is then connected to the VCA. For more flexibility, no CV connections for the Bullfrog are present on the voice card, so you have to do the patching yourself.