One of the few downsides to the Famicom AV, compared to the Famicom, is the lack of a microphone on controller two. It is a really small downside, but there are games that use the microphone (Zelda, Palutena no Kagami) or require it to complete a portion of the game (Takeshi's Challenge).
The microphone, when "used", did two things. First, it mixed the sound with the audio output from the Famicom so you could hear yourself from the TV speakers. Second, it showed up as a button input on $4016 D2. Games can read D2 to determine whether something has been said into the microphone.
Now the Famicom AV condensed three logic chips, the two 40HC368s and the 74LS139, found in the Famicom into a single chip called the Nintendo JIO BU3720S. Its pinout can be found here : http://gammy.void.nu/nesrgb/NES_pio_pinout.txt
As you can see, this chip incorporates only enough logic to support the two controller ports and the expansion port. $4016 D2 is found on neither. However, it could be implemented by adding a 40HC386 and wiring one of its gates it as shown in this schematic : http://nesdev.com/Ntd_8bit.jpg The microphone could also be mixed with the signal from the audio output pins as shown on that schematic, just before the 1μF Capacitor, which I believe is C3 on the Famicom AV.
Of course, there are two practical considerations here. First, you need to find a way to route a wire to a microphone. You could drill a hole in the Famicom AV or perhaps push the wire through one of the vents. The second issue deals with the microphone itself. I do not know the electrical characteristics of the original microphone, and it apparently requires a 4069 Hex Inverter, a capacitor, a volume slider and a resistor to work. It looks like a piezoelectric microphone : http://www.retrofixes.com/2013/10/clean ... ixing.html
The microphone, when "used", did two things. First, it mixed the sound with the audio output from the Famicom so you could hear yourself from the TV speakers. Second, it showed up as a button input on $4016 D2. Games can read D2 to determine whether something has been said into the microphone.
Now the Famicom AV condensed three logic chips, the two 40HC368s and the 74LS139, found in the Famicom into a single chip called the Nintendo JIO BU3720S. Its pinout can be found here : http://gammy.void.nu/nesrgb/NES_pio_pinout.txt
As you can see, this chip incorporates only enough logic to support the two controller ports and the expansion port. $4016 D2 is found on neither. However, it could be implemented by adding a 40HC386 and wiring one of its gates it as shown in this schematic : http://nesdev.com/Ntd_8bit.jpg The microphone could also be mixed with the signal from the audio output pins as shown on that schematic, just before the 1μF Capacitor, which I believe is C3 on the Famicom AV.
Of course, there are two practical considerations here. First, you need to find a way to route a wire to a microphone. You could drill a hole in the Famicom AV or perhaps push the wire through one of the vents. The second issue deals with the microphone itself. I do not know the electrical characteristics of the original microphone, and it apparently requires a 4069 Hex Inverter, a capacitor, a volume slider and a resistor to work. It looks like a piezoelectric microphone : http://www.retrofixes.com/2013/10/clean ... ixing.html