mindTunes
FPGA mp3/wav memo recording
Jonathan Chen, Po-Han Huang, Michael Kempf, Yen-Liang Tung, Christos Vezyrtzis
Object – Environment of System
Music/voice signals are
band-limited signals
=
t
Memo storing dictates
capacity instead of
quality as primary
concern
We need:
Low complexity system
Widely accepted storage format and medium
Therefore:
Minimize sampling rate (8kHz) and bit analysis (16 bit)
Choose wav/mp3 and USB Flash Drive
Hardware Setup (i-side only)
Programmable A/D converter
1-bit output stream : Use S/P
8kHz sampling rate, dual
channel
No pre-filtering
Same setting for D/A
Why only input? Symmetry
Output of hardware: at request of the processor / encoder
Very quick sample demand, very slow sampling
Solution: HARDWARE MUST SLOW DOWN THE SOFTWARE
How long? As much as it takes !!
S/P & Clock Setup (FYI)
Clocks are always created as fractions of the Audio Clock: 18.432MHz
18.432MHz / 2304 = 8kHz (Sampling)
A/D Output must be converted to 16-bit word before the next sample
18.432MHz / 144 = 128kHz (S/P)
Within 1-sample time interval we create one 16-bit word (sample)
FIFO
 Processor much faster than A/D
Within 1 sample time : 6250 read commands but only 1 new sample !!
Pause processor until there
is word to read from the
FIFO
In the process of reading:
If there is a read
command pause the
processor
Put the word on the
bus
Then read it!!
3 cycle delay (w.r.t.
processor clock)
FIFO(2) (FYI)
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Questions:
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How big? mp3 frame
Synchronous? NO Moore
state machine
Input & Output FIFO
equally spaced ? NO
Delay between successful
reading and output word:
3 cycles
FIFO(3)
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Operation:
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Altera MegaFunction
One clock cycle after rdreq,
data is presented at output
q; usedw decrements
One clock cycle after wrreq,
data is latched in; usedw
increments
Underflow circuitry prevents
data being read from an
empty FIFO; FIFO blocks
Overflow circuitry prevents
data being wrteen to a full
FIFO; FIFO blocks
Timing Diagram (Input) (FYI)
(1) Read & Write are deassetred  WAITREQUEST
(3) FIFO is not empty
 prepares for output
(4) FIFO is not empty
& Read command 
data is on the bus
(5) WAITREQUEST 0 and data is read
(2) New sample arrives at the FIFO  FIFO is ready for output
Operating System


uClinux – microcontroller implementation
of Linux.
Enables the incorporation of device
drivers – without having to write them!
Storage/File System

Utilized USB mass storage device

Running as a host device on DE2

Once mounted, provides convenient
storage of recordings in a “directory”
I/O programming in user space
 Not kernel space - no interrupts
 Can’t use Hardware Abstraction
Layer (e.g. IORD16_Direct)
 To access peripherals (switches,
audio) use memory pointer access
WAV Format Recording
The ‘RIFF’ chunk descriptor
• WAVE format
The ‘fmt’ sub-chunk
• format of the sound information
The ‘DATA’ sub-chunk
• size of the sound information
and raw sound dat
MP3 Format (MPEG 1 Layer3)

MP3 is composed of succession of
frames

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i.e. frame1 - frame2 - frame3 -……..
Each frame is totally independent, can
cut any part of MPEG files and play
correctly.
MP3 Frame Header
Choosing MP3 version
Bits
MPEG1
MPEG2
MPEG2.5 (ext 2)
00
44.1kHz
22kHz
11kHz
01
48kHz
24kHz
12kHz
10
32kHz
16kHz
8kHz
11
reserved
reserved
reserved
•Since we want to user low sampling Rate - 8kz, we choose MPEG 2.5
Layer3
•Note: The only difference MPEG 1,2,2.5 is MPEF 2 and 2.5 support more bit
rate, and sampling rate. Everything else are the same.
MP3 Header in our case
Length (bits)
Description
In our case
Bits
11
Frame Sync
Always 1
11111111111
2
MPEG Version
2.5
00
2
Layer
Layer3
01
1
CRC
no
0
4
Bit rate
128k bits/sec
1200
2
Sampling Rate
8khz
10
1
Padding
yes
1
1
Private bit
Only informative
1
2
Channel
Mono
11
2
Mode ext
Only used in Stereo 11
1
Copyright
no
0
1
Original
no
0
2
Emphasis
no
0
Encoding Implementation
ADC

.Wav file
At USB
Encoding
.mp3
file at
USB
The reason to have .wav file in the
transition is that the mp3 encoding
algorithm is to slow to make it realtime
Encoding Implementation
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Take a code from shine fixed point version
1.09 which was originally used on ARM
machine
Strip out the part about ARM and change the
part of assembly code to C in order to run
under nios-uclinux
Made it a function for main code to call
Decoding: Uclinux – MP3PLAY
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Support MPEG-1, 2, 2.5 Layer 1 2 3
Support both fixed-point and floating-point
Can use Assembly to optimize the
computing
Implementation:
Set to use fixed-point function in mp3dec library and do not use
Assembly optimization
Write out PCM data to FIFO input
Add header on PCM data to write to WAV
Decoding Implementation
.mp3
file at
USB
Decoding
DAC
.Wav file
At USB
Problems – Comments
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Software(Decoding):
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Now, we can hear artist’s vocal in the background with lots of white
noise
Before waitrequest function enable, it sounded like machine gun
With waitrequest enabled, artist’s voice sounds retarded
After we increased the FIFO size, the tempo is close to original clip
Write to wav file, but cannot hear sound
Write raw PCM data, listening on Matlab does not work
Software(Encoding):
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Too slow to be real-time
Inline assembly optimization
Hardware mul() implementation
Problems – Comments
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Hardware:
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FIFO timing
 Need to synchronize bus clock (100 MHz) and S. CLK (8 kHz)
 Encoder/Decoder might be consuming/feeding data faster than ADC/DAC
can process!
 Need to increase size of FIFO
Asynchronous “wait”
 waitrequest is combinational logic; state machine is sequential
Choice of Different configurations
Size of FIFO’s (mp3 frame)
Simulation :
 Initial sample of FIFO undefined at output
 Caused by Legacy mode (chosen as a suggestion of the software)
Thanks for help/listening